THE    TREASURES 
OF   COAL  TAR. 

ALEXANDER  FIND  LAY 


THE  TREASURES 
OF  COAL  TAR 


COAL  TAR  TREE  CHART 

Illustrating  the  various  chemical  products  de- 
rived from  Coal  and  Coal  Tar,  designed  in 
the  form  of  a  Genealogical  Tree.     34"  x  36". 
Revised  Edition. 

BY  WALLACE  C.  NICKELS,  F.C.S. 


THE  TREASURES 
OF  COAL  TAR 


BY 

ALEXANDER  FINDLAY 

1 1 
M.A.,  D.Sc.,  F.I.C. 

PROFESSOR   OF   CHEMISTRY  IN   THE   UNIVERSITY  OF   WALES 

AND   DIRECTOR   OF  THE   EDWARD   DAVIES   CHEMICAL   LABORATORIES 

UNIVERSITY  COLLEGE  OF   WALES,    ABERYSTWYTH 

AUTHOR  OF 

1  CHEMISTRY  IN   THE   SERVICE   OF   MAN  ' 
ETC. 


WITH  THREE  FIGURES  IN  THE  TEXT 


NEW  YORK 
D.  VAN  NOSTRAND  COMPANY 

1917 


Printed  in  Great  Britain 
fy  TurnbuZl  &>  Sftars,  Edinburgh 


s* 
T 


TO 

MY  MOTHER 


S820G8 


PREFACE 

IN  order  that  the  effort  now  being  made  to  promote 
the  more  widespread  application  of  science,  and 
more  especially  to  render  this  country  independent 
of  others  for  the  supply  of  the  dyes  necessary  for 
the  maintenance  of  our  textile  industry,  shall  not 
be  relaxed,  it  is  essential  that  the  people  as  a  whole 
should  interest  themselves  in  the  work,  and  should 
gain  some  knowledge  of  what  has  been  achieved 
in  the  past,  and  some  understanding  of  the  nature 
and  complexity  of  the  problems  to  be  solved.  As 
the  matter  is  urgent,  and  of  vital  importance  for 
the  welfare  of  this  country,  the  writer  felt  that, 
even  in  a  time  of  much  preoccupation,  he  could 
not  refuse  the  invitation  of  the  publishers  to  dis- 
cuss in  a  readily  intelligible  manner  the  production 
and  utilisation  of  coal  tar,  and  to  indicate,  suffi- 
ciently fully  for  the  general  reader,  the  almost 
infinite  variety  of  materials — dyes,  drugs.,  perfumes, 
explosives — for  the  manufacture  of  which  coal  tar 
is  the  raw  material. 

Based  on  this  invaluable  by-product  of  the  manu- 
facture of  coke  and  of  illuminating  gas,  an  industry, 
or  rather  a  series  of  industries,  has  been  developed  ; 

vii 


viii      THE  TREASURES  OF  COAL  TAR 

but  although  Great  Britain  played  a  predominant 
part  in  the  early  stages  of  this  industrial  develop- 
ment, she  failed  to  retain  the  great  advantages 
she  had  gained,  and  the  manufacture  of  synthetic 
dyes  and  drugs  became  increasingly  a  German 
monopoly.  To  such  an  extent  was  this  the  case 
that,  before  the  outbreak  of  war,  Germany  was 
producing  more  than  three  times  the  quantity  of 
coal-tar  products  produced  by  all  the  rest  of  the 
world  combined.  It  is  true  that  dyes  were  manu- 
factured in  considerable  amount  in  this  country, 
but  our  manufacturers  rested  content,  in  too  great 
a  measure,  with  their  dependence  on  German 
"  intermediates,'^  which,  instead  of  making,  they 
imported  and  worked  up  into  dyes.  In  spite  of 
the  warnings  uttered  by  our  foremost  chemists 
during  the  past  thirty  years,  in  spite  of  the  object- 
lessons  furnished  by  the  destruction  of  the  European 
madder  and  the  decay  of  the  Indian  indigo  planta- 
tions, this  country  failed  to  develop  its  coal-tar 
chemical  industries  on  a  national  scale ;  and  as 
a  result  of  this  failure  she  found  herself,  on  the 
outbreak  of  war,  placed  in  a  position  of  great 
gravity.  Perilously  handicapped  in  our  production 
of  the  munitions  of  war,  threatened  with  the  de- 
struction of  our  textile  industry  through  the  cutting 
off  of  the  supply  of  the  German-made  intermedi- 
ates and  dyes,  and  with  the  health  of  our  people 
and  army  endangered  through  shortage  of  those 


PREFACE  ix 

synthetic  drugs  with  which  the  German  chemical 
industry  had  supplied  us,  we  were  brought  face  to 
face  with  our  past  neglect  of  chemical  science  and 
with  our  failure  to  encourage  the  application  of 
that  science  in  our  industries.  The  chemists  of 
the  country  were  hurriedly  mobilised,  and  the 
production  of  the  essential  munitions  of  war  and 
of  a  sufficiency  of  drugs  was  ensured ;  the  Govern- 
ment came  to  the  help  of  the  dye-making  industry, 
and  in  the  past  two  years,  in  spite  of  many  handicaps, 
great  progress  has  been  made. 

But  what  of  the  future  ?  Can  we  feel  sure  that 
the  lessons  of  the  war  have  been  learned  and  that 
the  loss  and  bitter  experience  of  the  past  three 
years  will  be  turned  to  permanent  gain  ?  Have 
our  people  acquired  that  new  outlook,  that  new 
mentality,  which  is  the  only  safeguard  of  our 
future  ?  As  I  have  written  elsewhere,  we  are  all 
prone  to  blame  our  manufacturers  and  directors  of 
industry,  and  to  place  on  them  the  responsibility 
for  our  backwardness  in  the  recognition  of  the 
value  of  science,  but  we  have  to  remember  that  they 
are  themselves  but  a  part  of  the  national  system, 
and  their  organisation  and  outlook  an  expression 
of  the  national  character  and  habit.  Until  we 
realise  that  our  present  unfortunate  position  is 
the  result  of  a  national  defect,  which  shows  itself 
most  glaringly  in  our  lack  of  interest  in  education 
and  of  a  desire  for  knowledge,  and  until  we  realise 


x         THE  TREASURES  OF  COAL  TAR 

the  necessity  for  each  and  all  of  us  gaining  a  new 
standpoint  and  outlook,  gaining  a  new  ideal,  we 
cannot  hope  for  a  permanent  improvement  in  the 
attitude  of  the  country  and  manufacturers  towards 
science  and  its  applications.  Lord  Moulton  has 
quoted  the  words  of  a  German  industrial  chemist : 
"  England  talks  now  not  only  of  holding  her  own 
in  war,  but  beating  us  in  our  chemical  industries. 
She  cannot  do  it,  and  that  is  because  the  nation 
is  incapable  of  the  moral  effort  to  take  up  an 
industry  like  that — which  implies  study,  which  im- 
plies concentration,  which  implies  patience,  which 
implies  fixing  one's  eye  on  the  distant  consequences 
and  not  considering  merely  the  momentary  profit." 
That  is  a  challenge  which  this  country  cannot  refuse 
to  take  up,  but,  in  taking  it  up,  let  us  realise  that 
success  can  be  achieved  only  by  a  more  general 
appreciation  of  science,  by  the  cultivation  and 
encouragement  of  chemical  research  in  an  enor- 
mously higher  degree  than  in  the  past,  and  by 
the  continual  co-operation  between  science  and 
technology.  And  it  is  important,  also,  to  realise 
that  it  is  not  merely  science  in  its  immediate 
applications  to  industry  that  we  must  cultivate 
and  encourage,  but  also,  and  more  especially,  pure 
science  or  "  experimental  research  motivated  solely 
by  the  desire  to  increase  knowledge."  The  ac- 
quisition of  knowledge  must  precede  its  applica- 
tion ;  chemical  invention  must  follow  chemical 


PREFACE  xi 

discovery.  All  the  great  discoveries,  all  the  great 
advances  have  been  made,  not  as  a  result  of  effort 
to  achieve  results  of  immediate  industrial  im- 
portance, but  as  the  result  of  a  patient  and  per- 
severing pursuit  of  knowledge.  In  developing  the 
coal-tar  industries  we  can  succeed  if  we  will ;  let 
us  will. 

One  point  more  must  be  borne  in  mind.  Coal 
tar  is  produced  not  as  a  primary  but  as  a  by-pro- 
duct in  the  manufacture  of  illuminating  gas  and 
of  metallurgical  coke.  Its  production  is  therefore 
dependent  on  the  demand  for  coal  gas  and  for  coke, 
and  the  outlet  for  the  latter  depends  on  the  develop- 
ment of  our  metallurgical  industries.  A  proper 
balance  between  output  of,  and  the  profitable 
outlet  for,  the  different  products  and  by-products 
of  the  distillation  of  coal  must  be  established ; 
and  the  whole  series  of  interdependent  and  inter- 
locking chemical  industries  must  be  carefully  organ- 
ised and  developed  so  as  to  ensure  the  greatest 
efficiency  and  best  utilisation  of  all  the  products. 
The  question  of  the  production  and  utilisation  of 
coal  tar  and  coal-tar  products  is  one  of  great  com- 
plexity as  it  is  also  one  of  great  economic  import- 
ance ;  and  it  must  be  treated  as  part  and  parcel 
of  the  much  larger  question  of  the  most  effective 
utilisation  of  our  national  reserves  of  coal. 

My  thanks  are  due  to  Messrs  Longmans,  Green 
&  Co.  for  the  use  of  the  block  of  Figure  3,  taken 


xii       THE  TREASURES  OF  COAL  TAR 

from  my  "  Chemistry  in  the  Service  of  Man  "  ; 
to  the  Comptroller-General  of  the  Department  of 
Commercial  Intelligence  of  the  Board  of  Trade 
for  particulars  regarding  the  production  of  coal 
tar ;  and  to  Mr  C.  M.  Whittaker,  of  British  Dyes, 
Ltd.,  for  information  regarding  dyes.  I  am  also 
indebted  to  my  wife  for  her  assistance  in  passing 
the  book  through  the  press. 

A.  F. 


Y  GLYN,  LLAN PARIAN, 

NR.  ABERYSTWYTH,  CARDIGANSHIRE, 

September  1917. 


CONTENTS 


CHAPTER  I 

THE   PRODUCTION   OF   COAL  TAR  ....  I 

CHAPTER  II 

THE  DISTILLATION   OF  COAL  TAR         .  .  .  .  13 

CHAPTER  III 

THE  CONSTITUENTS  OF  COAL  TAR  AND  THEIR  APPLICA- 
TIONS  IN  THE  RAW  STATE  .  .  .  .  22 

CHAPTER  IV 

MOLECULAR  ARCHITECTURE        .  ....  31 

CHAPTER  V 

THE  PRODUCTION  OF  DYES  FROM  COAL  TAR    .     .     48 

CHAPTER  VI 

AZO-DYES 70 

CHAPTER  VII 

ANTHRACENE  DYES  AND   VAT  DYES  80 

xiii 


xiv      THE  TREASURES  OF  COAL  TAR 

PAGE 

CHAPTER  VIII 

INDIGO  AND  ITS  DERIVATIVES  ....  90 

CHAPTER  IX 

DRUGS,   PERFUMES,  AND   PHOTOGRAPHIC   DEVELOPERS  IOO 

CHAPTER  X 
EXPLOSIVES 122 

INDEX 133 


THE  TREASURES  OF 
COAL  TAR 

CHAPTER  I 

THE   PRODUCTION   OF  COAL  TAR 

COAL,  the  fossilised  and  more  or  less  completely 
carbonised  remains  of  the  luxuriant  vegetations 
of  a  long  bygone  age,  forms  at  once  the  source  of 
much  of  our  material  wealth  and  the  basis  on  which 
our  industrial  and  commercial  prosperity  has  been 
reared  during  the  past  hundred  years.  The  coal 
mines  of  this  country,  worked  at  least  as  early  as 
the  thirteenth  century,  have  since  that  time  pro- 
vided us,  in  ever-increasing  amounts,  with  a  valu- 
able fuel  both  for  domestic  and  industrial  purposes. 
The  introduction  of  coal,  especially  as  a  domestic 
fuel,  was  for  a  long  time  regarded  with  disfavour, 
and,  even  in  the  seventeenth  century,  met  with 
an  active  boycott  on  the  part  of  "  the  nice  dames 
of  London,"  who  "  would  not  come  into  any  house 
or  rooms  where  sea-coales  were  burned,  nor  willingly 
eat  of  meat  that  was  either  sod  or  roasted  with  sea- 
coale  fire  " — doubtless  by  reason  of  the  pollution 
of  the  atmosphere  by  smoke  and  of  the  stench 


2          THE  TREASURES  OF  COAL  TAR 

produced  by  the  burning  coal.  At  the*  present 
time,  however,  the  normal  annual  consumption  of 
coal  in  this  country  amounts  to  about  190,000,000 
tons,  of  which  about  40,000,000  tons  are  consumed 
for  domestic  heating. 

But  although  it  is  on  its  use  as  a  fuel,  as  a 
reservoir  of  energy  derived  from  the  sunlight  of  a 
long-distant  past,  that  the  material  comfort  and 
well-being  of  the  people  so  largely  depend,  coal 
yet  conceals  within  itself  another  wealth — long 
squandered  through  ignorance  and  even  now  but 
partially  utilised — which  the  wizardry  of  Science 
has  discovered  and  made  available  only  within 
comparatively  recent  years.  And  it  is  of  this 
wealth,  or  part  of  this  wealth,  derived  from  the 
chemical  transformation  of  coal  and  from  the  black 
and  viscid  fluid,  the  coal  tar,  produced  by  the  "  de- 
structive distillation  "  of  coal,  that  it  is  the  purpose 
of  this  book  to  treat. 

Although  coal  had  been  used  as  a  fuel  as  early 
as  the  beginning  of  the  fourteenth  century,  it  was 
not  till  near  the  end  of  the  seventeenth  century 
that  the  distillation  of  coal  in  closed  vessels  was 
carried  out,  the  first  English  patent  being  granted 
in  1681  to  J.  J.  Becher  and  Henry  Serle  for  "  a  new 
way  of  making  pitch  and  tarre  out  of  pit-coale, 
never  before  found  out  or  used  by  any  other."  At 
first  an  industry  of  very  small  proportions,  it  was 
not  till  the  early  years  of  the  nineteenth  century 


THE  PRODUCTION  OF  COAL  TAR         3 

that  the  distillation  of  coal  began  to  be  carried 
out  extensively,  and  then  not  for  the  purpose  of 
producing  tar  and  pitch,  but,  primarily,  for  the  pro- 
duction of  coal  gas  or  illuminating  gas.  Although 
the  production  of  an  inflammable  gas  from  coal 
had  long  been  known,  it  was  not  till  near  the  end 
of  the  eighteenth  century  that  the  Scotsman,  William 
Murdoch,  developed  the  process  for  the  production 
of  an  illuminating  gas  for  general  use.  Murdoch 
was  for  long  associated  with  the  engineering  firm 
of  Messrs  Boulton  &  Watt,  Birmingham,  and  it 
was  in  their  works  at  Soho  that  coal  gas  was  first 
used  (in  1798)  on  a  large  scale  as  an  illuminant. 
It  was,  however,  only  at  a  considerably  later  date 
that  coal  gas  came  into  general  use  for  the  lighting 
of  streets  and  public  buildings. 

When  ordinary  or  bituminous  coal  is  subjected 
to  "  destructive  distillation  "  by  heating  in  retorts 
out  of  contact  with  air,  there  are  produced  :  (i)  the 
combustible  gas  which  we  use  for  illuminating  and 
heating  purposes  ;  (2)  a  watery  liquor  containing 
ammonia,  derived  from  nitrogen  compounds  con- 
tained in  the  coal ;  (3)  a  thick,  dark-coloured  liquid, 
coal  tar  ;  (4)  coke,  which  remains  as  a  solid  residue 
in  the  retorts.  In  the  manufacture  of  illuminating 
gas  the  coal  is  heated  in  large  fire-clay  retorts  at 
a  temperature  of  about  1000°  C.  (about  1830°  F.), 
and  the  products  of  decomposition  are  led  away  by 


4          THE  TREASURES  OF  COAL  TAR 

a  pipe  the  mouth  of  which  dips  under  the  surface 
of  water  contained  in  what  is  known  as  the  hydraulic 
main  (Fig.  i).  Here  part  of  the  water  and  of  the 
coal  tar  condenses,  while  the  gaseous  products  pass 
away  to  a  series  of  cooling  pipes,  exposed  to  the  air, 
in  which  a  further  condensation  of  water  vapour 
and  of  tar  takes  place.  The  ammonia  present 
the  gas  dissolves  for  the  most  part  in  the  water 
produced,  and  the  remainder  is  removed  by  passing 
the  gas  through  "  scrubbers."  In  this  process  the 
ammoniacal  liquor,  coal  tar,  and  coke  are  merely 
by-products,  spoken  of  as  "  residuals " ;  and 
although  the  coke  has  always  been  a  by-product 
of  considerable  value  which  materially  affected 
the  price  of  the  illuminating  gas,  the  ammoniacal 
liquor  and  coal  tar  were  for  a  number  of  years 
regarded  as  waste  products  of  a  disagreeable  kind, 
the  disposal  of  which  involved  not  a  little  expense, 
and  thereby  retarded  to  some  extent  the  develop- 
ment of  the  gas-producing  industry.  This  con- 
dition of  affairs,  however,  has  been  entirely  changed, 
largely  owing  to  the  development  of  the  great 
chemical  industries  which  find  their  raw  material 
in  coal  tar,  as  well  as  to  the  increased  and  increasing 
employment  of  ammonia  compounds  as  fertilisers 
in  agriculture,  in  the  production  of  explosives, 
dyes,  and  soda  (by  the  Solvay  process),  and  in  many 
other  industries.  In  1913,  out  of  a  total  produc- 
tion from  all  sources  of  432,000  tons,  the  gas-works 


FIG.  i. — DIAGRAM  OF  GAS-MANUFACTURING  PLANT. 

A,  retort  in  which  coal  is  heated. 

B,  the  hydraulic  main. 

C,  outlet  for  the  tar. 

D,  gas  pipe. 

E,  tank  in  which  the  ammoniacal  liquor  collects. 

F,  cooling  pipes. 


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THE  PRODUCTION  OF  COAL  TAR    .     5 

of  this  country  produced  182,180  tons  of  sulphate 
of  ammonia,  the  value  of  which,  in  some  cases, 
amounted  almost  to  the  cost  of  the  coal  distilled ; 
while  the  production  of  coal  tar  in  gas-works 
amounted,  in  1910,  to  830,000  tons.  In  all,  about 
20,000,000  tons  of  coal  are  now  distilled  annually  in 
this  country,  primarily  for  the  production  of  coal  gas. 
But  it  is  not  only  for  the  production  of  gas  that 
coal  is  now  distilled.  Even  by  the  middle  of  the 
eighteenth  century  coke  had  to  a  large  extent  dis- 
placed wood  charcoal  and  was  very  generally  em- 
ployed in  the  smelting  of  iron ;  and  as  our  iron 
industry  advanced  and  extended,  so  also  the  dis- 
tillation of  coal,  primarily  for  the  production  of 
the  hard  and  dense  coke  required  for  metallurgical 
purposes,  became  an  industry  of  ever-increasing 
importance.  For  many  years  the  coking  of  coal 
was  carried  out  in  "  beehive  "  ovens,  which  are,  as 
the  name  implies,  chambers  of  beehive  shape  lined 
with  firebrick.  From  seven  to  eight  feet  high  and 
about  twelve  feet  in  diameter,  these  ovens  are  now 
generally  arranged  side  by  side  in  two  rows  so  as 
to  economise  heat  and  allow  of  the  hot  products 
of  distillation  being  carried  off  through  a  central 
flue  for  the  raising  of  steam  (Fig.  2).  Through  a 
hole  in  the  top,  the  oven,  still  hot  from  previous 
use,  is  charged  with  coal  to  a  depth  of  about  three 
feet,  the  door  of  the  oven  being  temporarily  bricked 
up.  Air  in  regulated  amount  is  admitted  to  the 


6          THE  TREASURES  OF  COAL  TAR 

space  above  the  coal,  where  combustion  of  the 
evolved  vapours  takes  place,  and  coking  or  carbon- 
isation proceeds  steadily  from  above  downwards, 
owing  to  the  heat  reflected  from  the  roof  and  walls 
of  the  oven.  In  this  process  the  heat  required  for 
the  coking  of  the  coal  is  derived  from  the  combustion 
of  the  gases  distilled  from  the  coal  as  well  as  from 
the  combustion  of  part  of  the  charge. 

Although,  in  these  beehive  ovens,  a  hard,  dense 
coke,  admirably  suited  for  metallurgical  purposes, 
is  produced,  the  process  is  a  most  wasteful  one, 
because  not  only  is  a  certain  amount  of  coal  lost 
through  combustion,  but  all  the  volatile  products 
of  distillation,  amounting  to  about  one-third  of 
the  weight  of  the  coal,  are  lost.  In  the  early  days 
of  the  industry,  when  there  was  little  or  no  outlet 
for  these  products,  when,  at  least,  their  commercial 
value  was  small  compared  with  that  of  the  coke, 
the  consciousness  of  waste  was  scarcely  awakened, 
and  the  coke  producers  made  no  attempt  to  recover 
and  utilise  the  by-products  of  the  coking  process. 
Moreover,  the  introduction  of  coking  ovens  which 
allow  of  the  recovery  of  the  volatile  by-products  of 
distillation  was  resisted  by  the  iron-makers,  as  it 
was  thought — not,  at  first,  without  reason — that 
the  coke  produced  in  them  was  inferior  to  that 
produced  in  the  old  beehive  ovens.  But  the  pre- 
judice which  for  long  existed,  more  especially  in 
Great  Britain,  has  now  been  proved  to  be  ground- 


THE  PRODUCTION  OF  COAL  TAR         7 

less,  and  the  increasing  importance  of  ammonia 
and  coal  tar,  and  the  necessity  for  greater  industrial 
economy,  are  leading  more  or  less  rapidly  to  the 
abolition  of  the  old  beehive  oven.  Whereas  in 
Great  Britain  in  1900  only  ten  per  cent,  of  all  the 
metallurgical  coke  was  produced  in  by-product 
recovery  ovens,  in  1913  about  sixty  per  cent,  was 
so  produced.  In  other  words,  of  the  20,000,000 
tons  of  coal  converted  into  coke,  about  13,500,000 
tons  were  coked  in  by-product  recovery  ovens  and 
6,500,000  tons  in  beehive  ovens.  In  the  United 
States,  similarly,  about  one-third  of  the  metal- 
lurgical coke  was  produced,  in  1914,  in  beehive 
ovens,  without  recovery  of  the  by-products,  where- 
as in  Germany,  in  1909,  only  about  one-fifth  of 
the  coke  was  produced  by  this  wasteful  process. 
Although,  in  this  respect,  this  country  has  lagged 
considerably  in  the  rear  of  Germany,  fairly  rapid 
progress  towards  a  more  economical  utilisation  of 
our  national  resources  in  coal  is  being  made ;  and 
this  will  doubtless  be  accelerated  by  the  experiences 
of  the  past  three  years  and  the  necessities  of  the 
future.  Although  the  initial  cost  of  the  by-product 
recovery  ovens  is  greater  than  in  the  case  of  the 
beehive  ovens,  it  would  appear,  from  evidence 
given  before  a  Royal  Commission  on  Coal  Supplies 
in  England,  that  the  value  of  the  by-products  would 
not  only  provide  a  profit  on  the  working  of  the 
plant,  but  would  also,  within  ten  years,  pay  off  the 


8          THE  TREASURES  OF  COAL  TAR 

capital  outlay.  At  the  same  time  it  has  to  be 
borne  in  mind  that  in  the  future  the  successful 
development  of  the  coking  industry,  with  recovery 
of  the  by-products,  must  very  largely  depend  on 
the  development  of  all  those  closely  interdependent 
industries — more  especially  chemical  industries — 
which  afford  a  remunerative  outlet  for  the  by-pro- 
ducts of  distillation,  and  it  is  of  the  highest  import- 
ance that  this  country  shall  make  a  determined 
and  well-directed  effort  towards  this  end. 

The  recognition  of  the  importance  of  recovering 
the  volatile  matter  produced  in  the  coking  of  coal 
has,  during  the  past  thirty  or  forty  years,  led  to 
great  activity  in  the  work  of  designing  and  construc- 
tion of  ovens  adapted  for  the  purpose,  and  several 
different  types  are  at  present  in  use.  In  one  type, 
a  modification  of  the  Coppee  oven,  for  example, 
the  oven  is  formed  by  a  chamber  about  thirty  feet 
in  length,  two  feet  wide,  and  five  feet  high,  heated 
by  means  of  gas.  A  number  of  these  ovens  are 
generally  arranged  side  by  side  and  are  charged 
from  hoppers  which  run  on  rails  over  the  series  of 
chambers.  As  the  coking  proceeds,  the  volatile 
by-products  pass  away  through  pipes  to  a  hydraulic 
main  and  condensers,  where  the  ammoniacal  liquor 
and  the  tar  are  collected  ;  and  the  gas  is  then  passed 
to  gas-holders  whence  it  is  drawn  off  as  required 
and  used  for  heating  the  ovens.  Since,  with  im- 
proved construction,  considerably  more  gas  is  pro- 


THE  PRODUCTION  OF  COAL  TAR         9 

duced  than  is  required  for  this  purpose,  the  coke 
ovens  are  now  a  valuable  source  of  gas  supply,  the 
excess  gas  being  employed  for  heating,  for  the  pro- 
duction of  power,  and  even  for  illuminating  pur- 
poses. Thus  the  city  of  Leeds,  for  example,  takes 
a  million  cubic  feet  per  day  of  coke-oven  gas 
from  the  Middleton  Estate  and  Colliery  Company, 
this  gas  being  then  "  enriched  "  with  carburetted 
water-gas. 

By  the  introduction  of  these  ovens  great  economies 
have  been  effected  owing  to  an  increase  in  the  yield 
of  coke — upwards  of  seventy  per  cent,  of  the  weight 
of  coal  being  obtained  as  coke — and  to  the  recovery 
of  the  very  valuable  by-products,  ammonia  and 
coal  tar. 

Coal,  it  must  be  borne  in  mind,  is  not  a  definite 
chemical  substance,  but  a  complex  mixture  of  sub- 
stances, the  nature  of  which  is  not  yet  definitely 
known,  and  doubtless  varies  considerably  in  the 
case  of  the  different  kinds  of  coal.  It  will  there- 
fore readily  be  understood  that  the  nature  of  the 
products  as  well  as  their  relative  amounts  depend 
on  the  kind  of  coal  distilled ;  and  they  depend, 
moreover,  in  a  very  marked  degree  on  the  general 
conditions  under  which  the  distillation  of  the  coal 
is  carried  out — for  example,  on  the  temperature, 
size  and  shape  of  the  retort,  and  on  the  time  during 
which  the  volatile  products  remain  in  contact  with 


io        THE  TREASURES  OF  COAL  TAR 

the  red-hot  walls  of  the  retort.  As  regards  the 
composition,  the  most  important  points  of  difference 
are  found  in  the  nature  of  the  so-called  hydrocarbons 
(compounds  of  carbon  and  hydrogen)  present  in 
the  tar.  .When  the  distillation  is  carried  out  at  a 
low  temperature  (say  about  450°  C.  or  840°  F.),  the 
tar  contains  mainly  hydrocarbons  belonging  to  the 
so-called  aliphatic  series  (p.  38),  suitable  for  use 
as  motor  spirit,  and  as  illuminants  and  lubricants 
(vaseline).  Tars  of  this  description  are  produced, 
for  example,  in  the  distillation  of  coal  for  the  produc- 
tion of  coalite,  in  the  manufacture  of  Mond  gas,  and 
in  blast  furnaces  (mainly  in  Scotland)  where  raw 
coal  is  used  in  place  of  coke.  When,  however,  the 
distillation  is  carried  out  at  a  high  temperature  (say 
about  1000°  C.  or  1830°  F.),  as  is  the  case  when 
coal  is  distilled  for  the  production  of  illuminating 
gas  or  of  the  hard  coke  used  for  smelting  and  other 
metallurgical  purposes,  the  prevailing  hydrocarbons 
are  those  belonging  to  the  "  aromatic  "  class  (p.  40), 
e.g.  benzene  and  its  derivatives.  It  is  this  kind  of 
coal  tar  which  is  of  such  importance  as  furnishing 
the  raw  materials  for  the  production  of  the  innumer- 
able dyes,  drugs,  perfumes,  explosives,  etc.,  the  pro- 
duction of  which  now  constitutes  such  an  imposing 
and  valuable  industry. 

Although  the  nature  of  the  tar  constituents,  as 
well  as  their  relative  amounts,  depends  on  the  con- 
ditions under  which  the  distillation  is  carried  out, 


THE  PRODUCTION  OF  COAL  TAR       n 

we  may  say  that  under  the  general  conditions  met 
with  in  gas  and  coking  works,  one  ton  of  dry  coal 
will  yield,  in  addition  to  11,000-12,000  cubic  feet 
of  gas,  20-35  Ibs.  of  sulphate  of  ammonia,  56-120  Ibs. 
of  coal  tar,  and  1400-1800  Ibs.  of  coke. 

Owing,  more  especially,  to  the  demand  for  metal- 
lurgical coke,  the  annual  production  of  coal  tar  has 
now  assumed  very  large  dimensions.  This  country 
has  always  occupied  a  foremost  place  in  the  gas- 
producing  industry,  and  she  was  also,  for  long,  the 
premier  producer  of  coal  tar.  But  it  is  probable 
that  by  1913  she  had  already  lost  to  Germany  her 
position  of  pre-eminence  in  the  tar-producing  in- 
dustry owing  to  the  great  development  in  that 
country  of  the  iron  and  steel  industry,  and  the 
consequent  demand  for  coke.  Moreover,  owing  to 
the  magnitude  of  the  German  chemical  industries, 
most  of  the  coal  tar  formed  in  the  production  of 
this  coke  was  recovered,  and  Germany  thereby 
made  herself  largely  independent  of  this  country 
for  the  supply  of  coal  tar,  of  which,  even  in 
1908,  she  imported  from  Great  Britain  40,000  tons. 
Although,  up  to  the  outbreak  of  war,  Germany 
imported  from  this  country  considerable  amounts 
of  anthracene  and  of  phenol  (carbolic  acid),  the 
development  of  the  synthetic  production  of  the 
latter  compound  from  benzene  will  doubtless 
render  its  continued  importation  unnecessary. 


12        THE  TREASURES  OF  COAL  TAR 

In  1901  the  approximate  annual  production  of 
coal  tar  throughout  the  world  has  been  estimated 
as  follows  : 

United  Kingdom       .         .  908,000  tons. 

Germany  .         .         .  590,000  ,, 

United  States   .         .         .  272,400  ,, 

France     ....  190,680  ,, 
Belgium,  Holland,  Sweden, 

and      other      European 

countries       .         .         .  199,760  ,, 

All  other  countries     .         .  227,000  ,, 


2,660,440     „ 

In  the  following  years  of  the  decade  the  amounts 
largely  increased,  as  shown  by  the  following  approxi- 
mate figures  : 

United  Kingdom  (1910)  1,380,000  tons. 

Germany  (1912)     .         .  1,082,197  ,, 

United  States  (1912)       .  564,000  ,, 

France  (1909)         .         .  214,800  „ 


CHAPTER  II 

THE   DISTILLATION   OF  COAL  TAR 

CRUDE  coal  tar,  as  it  is  obtained  from  gas  and  coking 
works,  although  it  may  vary  not  a  little  according 
to  its  origin,  is  a  thick,  oily,  dark-coloured  liquid 
rather  heavier  than  water  (specific  gravity  about 
1-2).  In  the  early  days  of  the  coal-distilling  in- 
dustry this  tar  was,  as  has  been  said,  a  disagreeable 
waste  product,  the  disposal  of  which  was  the  source 
of  much  worry  and  annoyance  to  the  producer  no 
less  than  to  the  general  public  in  the  neighbourhood. 
As  it  was  impossible,  by  reason  of  the  nature  of  the 
material,  to  get  rid  of  the  accumulations  of  tar  by 
running  it  into  streams  and  rivers,  the  difficulty 
of  its  disposal  was  solved,  to  some  extent,  by 
burning  the  tar  as  a  fuel.  A  certain  amelioration 
was  brought  about  by  the  use  of  coal  tar  as  a  paint 
for  wood  and  metal  work,  and  for  this  purpose  the 
more  volatile  portions  were  removed  by  distilla- 
tion, the  "  spirit "  so  obtained  being  used  either 
as  a  substitute  for  turpentine  in  making  varnishes 
or  as  a  solvent  for  rubber  in  the  manufacture  of 
a  waterproof  material  which  is  still  known  by 
the  name  of  the  original  Glasgow  manufacturer, 

13 


14        THE  TREASURES  OF  COAL  TAR 

Mackintosh.  Much  of  the  residue  from  the  dis- 
tillation was  burned  for  the  production  of  lamp- 
black, which  is  used  in  the  manufacture  of  pigments, 
blacking,  and  printer's  ink. 

Hitherto,  the  distillation  of  coal  tar  had  been 
carried  out  only  on  a  comparatively  small  scale, 
and  the  demand  for  tar  lagged  far  behind  the 
supply,  until,  in  1838,  an  entirely  new  situation 
was  created  through  the  introduction  of  a  process 
for  preserving  or  "  pickling  "  timber  (p.  27)  ;  and 
an  industry  which  has  now  attained  to  enormous 
proportions  was  thereby  inaugurated.  Moreover, 
in  the  year  1845  another  great  stimulus  was  given 
to  the  coal-tar  industry  owing  to  the  scientific  in- 
vestigations which  were  carried  out  by  Professor 
Hofmann  and  his  students  at  the  newly-founded 
Royal  College  of  Chemistry  in  London,  investiga- 
tions which  not  only  led  to  the  isolation  from  coal 
tar  of  some  of  its  main  constituents,  but  were  the 
roots  from  which  the  vast  modern  industry  of  coal- 
tar  dyes,  drugs,  and  explosives  has  really  grown. 
Owing  to  these  developments,  which  we  shall  dis- 
cuss more  fully  in  the  sequel,  a  demand  was  created 
for  the  more  volatile  portions  of  coal  tar  which 
had  been  rejected  by  the  timber-pickling  industry ; 
and  a  more  complete  utilisation  of  the  constituents 
of  the  tar  was  thereby  made  possible. 

Although,  in  the  past,  crude  coal  tar  was  largely 
employed  not  only  as  a  liquid  fuel  but  also  for  the 


THE  DISTILLATION  OF  COAL  TAR      15 

manufacture  of  roofing  felt,  the  tarring  of  roads, 
and  other  purposes,  the  water  and  ammoniacal 
liquor  present  in  the  tar  were  found  to  be  detri- 
mental, so  that  now  only  a  small  amount  of  tar  is 
used  in  the  crude  state,  except  in  those  cases  where 
it  is  employed  as  a  fuel.  By  far  the  greater  pro- 
portion of  the  coal  tar  is  now  subjected  to  a  process 
of  distillation,  a  process  first  carried  out  systematic- 
ally by  Charles  Blachford  Mansfield  1  in  1848. 

Coal  tar,  even  after  being  freed  from  the  water 
and  ammonia  with  which,  when  it  is  received  from 
the  gas  and  coke  works,  it  is  intimately  mixed,  is 
not  a  single  substance,  but  an  exceedingly  complex 
mixture  of  over  two  hundred  different  compounds, 
some  of  which  are  present,  however,  only  in  very 
minute  amount.  Although  the  complete  separa- 
tion and  isolation  of  all  these  different  substances 
is  a  matter  of  the  greatest  difficulty,  and  is  not 
attempted  in  practice,  it  is  possible,  by  subjecting 
the  tar  to  a  process  of  distillation,  to  separate  it 
into  a  number  of  portions  or  "  fractions  "  which 
distil  over  at  different  temperatures.  The  general 
principle  on  which  the  apparatus  employed  is  con- 

1  Mansfield  was  a  pupil  of  Hofmann,  and,  under  his  direction, 
was  the  first  to  separate  coal-tar  naphtha  into  its  constituents  by 
fractional  distillation.  Unfortunately,  in  1856,  while  carrying 
out  this  work,  the  contents  of  the  still  boiled  over  and  caught 
fire.  While  endeavouring  to  extinguish  the  flames  Mansfield 
was  so  severely  burned  that  death  supervened  in  a  few  days. 


16        THE  TREASURES  OF  COAL  TAR 

structed  is  illustrated  by  Fig.  3.  Here,  A  is  a  vessel 
or  "  still "  in  which  the  liquid  is  boiled  and  so  con- 
verted into  vapour  which  passes  through  the  long 
neck,  B,  to  a  spiral  "  worm  "  or  condenser,  C,  kept 
cool  by  means  of  flowing  water.  The  condensed 
vapour  issues  at  D  and  can  be  collected  in  a  "  re- 
ceiver/* E  is  a  tube  by  which  water  enters  and 
F  is  the  outlet  for  the  warm  condenser  water. 


FIG.  3. — APPARATUS  USED  FOR  DISTILLING  LIQUIDS. 

(Illustration  from  "  Chemistry  in  the  Service  of  Man.") 

T  is  a  thermometer  to  indicate  the  temperature 
of  the  liquid  in  the  still.  In  actual  practice  the  still 
consists  of  a  large  iron  boiler— capable  of  holding 
twenty  tons  or  more  of  tar — set  in  brickwork  and 
heated  by  a  fire.  Since  the  first  fractions  which 
distil  over  are  rather  volatile  liquids  at  the  ordinary 
temperature,  the  condensing  coil  is  cooled  by  means 
of  cold  water ;  but  as  the  distillation  proceeds  the 
substances  which  pass  over  solidify  on  cooling, 
and  so  the  condenser  is  kept  warm  by  means  of 


THE  DISTILLATION  OF  COAL  TAR      17 

hot   water  in   order   to   prevent   the   choking   of 
the  coil. 

By  this  process  of  distillation  the  tar  is  separated 
into  a  number  of  portions.  First  of  all,  while  the 
temperature  of  the  still  gradually  rises  to  170°  C. 
(338°  F.),  there  distils  over  a  light  inflammable 
liquid  known  as  "light  oil."  This  is  followed, 
between  the  temperatures  of  170°  C.  and  230°  C. 
(338°  F.  and  446°  F.),  by  the  "  carbolic  oils,"  so 
called  because  they  contain  the  main  portion  of  the 
carbolic  acid  present  in  the  tar.  At  still  higher 
temperatures,  between  230°  C.  and  270°  C.  (446°  F. 
and  518°  F.),  one  obtains  a  complex  mixture  of 
substances  constituting  the  "  creosote  oils  "  ;  and 
lastly  there  pass  over,  between  270°  C.  and  400°  C. 
(518°  F.  and  752°  F.),  the  "anthracene  oils,"  the 
most  important  constituent  of  which  is  the  hydro- 
carbon anthracene.  After  the  different  fractions 
have  passed  over  there  remains  in  the  still  a  residue 
of  pitch.  By  this  process  of  distillation  there  are 
obtained,  from  one  ton  of  tar,  approximately  : 

12  gallons  of  light  oils. 
20         ,,         carbolic  oils. 
17         „         creosote  oils. 
38         „         anthracene  oils. 
ii  hundredweight  of  pitch. 

The  crude  coal  tar  having  in  this  way  been  sepa- 
rated into  a  number  of  different  portions,  each  of 


18        THE  TREASURES  OF  COAL  TAR 

these  is  then  subjected  to  suitable  chemical  treat- 
ment and  to  repeated  distillation  in  order  to 
effect  a  more  complete  purification  and  separation 
into  the  different  constituents.  Thus  the  light  oils 
are  separated  into  "  crude  benzol," — consisting  of 
a  mixture  of  the  hydrocarbons,  benzene,  toluene, 
and  xylene, — "  solvent  naphtha  "  and  "  burning 
naphtha,"  consisting  of  xylene  and  similar  but  more 
complex  hydrocarbons.  To  obtain  pure  benzene 
and  toluene,  such  as  are  required  in  the  manu- 
facture of  dyes,  drugs,  and  explosives,  the  crude 
benzol  is  "  rectified "  by  distillation  in  a  special 
still.1 

In  order  to  avoid  confusion  it  may  be  stated 
that  the  hydrocarbons  now  known  to  British 
chemists  as  benzene  (not  to  be  confounded  with 
benzine  or  benzoline)  and  toluene,  were  formerly 
called  benzol  (or  benzole)  and  toluol,  and  these 
names  are  still  employed  commercially.  The  term 
benzol,  however,  is  also  applied  in  commerce  to 
various  mixtures  of  hydrocarbons,  different  grades 

1  It  may  be  mentioned  that  although  benzene  and  toluene 
were  formerly  obtained  solely  from  coal  tar,  considerable 
quantities  of  these  compounds  are  now  obtained  from  coke- 
oven  gas  by  "  scrubbing  "  it  with  creosote  oil.  Indeed,  this  is 
now  a  more  important  source  of  crude  benzol  than  coal  tar. 
Since  the  removal  of  these  hydrocarbons  diminishes  both  the 
illuminating  power  and  the  calorific  value  of  the  gas,  the  above 
process  is  not  applied  in  large  measure  to  ordinary  coal  gas, 
although,  at  the  present  day,  owing  to  the  exigencies  of  war, 
considerable  quantities  of  these  valuable  compounds  are  obtained 
from  this  source. 


THE  DISTILLATION  OF  COAL  TAR    19 

of  "  benzol "  being  produced  for  use  in  the  arts. 
Thus  we  have  the  various  grades  known  as  90  per 
cent.,  50  per  cent.,  and  30  per  cent,  benzol,  these 
terms  being  applied  to  liquids  of  which  90,  50,  or 
30  per  cent,  distils  over  at  temperatures  up  to  the 
boiling-point  of  water  (100°  C.  or  212°  F.).  The 
composition  of  these  three  grades  of  commercial 
benzol  is  shown  in  the  following  table  : 


90  per  cent, 
benzol. 

50  per  cent, 
benzol. 

30  per  cent, 
benzol. 

Benzene 

80-9 

45'4 

13-5 

Toluene 

14-9 

40'3 

73-4 

Xylene 
Impurities     . 

2'2 
2-0 

12-4 
1-9 

117 
1-4 

The  portion  of  the  tar  distillate  known  as  "  car- 
bolic oils  "  or  "  middle  oils  "  is  likewise  separated 
by  chemical  and  physical  treatment  into  its  chief 
constituents.  On  allowing  it  to  cool  down  there 
separates  out  from  it  a  considerable  quantity  of  a 
hydrocarbon  known  as  naphthalene,  and  the  re- 
sidual oil  is  then  sold  as  "  crude  carbolic  acid/'  for 
the  manufacture  of  disinfectants.  On  gently  heat- 
ing the  crude  naphthalene  it  sublimes  or  passes  into 
vapour  which,  on  cooling,  solidifies  in  the  form  of  large 
crystalline  flakes.  In  this  way  it  is  purified.  Much 
of  the  crude  carbolic  acid  also  is  refined  by  treat- 
ment with  alkali  and  acid  and  subsequent  distilla- 
tion. By  this  means  pure  carbolic  acid  or  phenol, 
as  it  is  called  by  chemists,  is  obtained,  together  with 


20        THE  TREASURES  OF  COAL  TAR 

a  mixture  of  three  similar  compounds  known  as 
cresylic  acids  or  cresols. 

The  "creosote  oils"  or  "heavy  oils"  consist 
of  a  number  of  different  compounds  which, 
however,  are  not  separated  from  each  other. 
These  oils  are  merely  "  fractionated "  in  accord- 
ance with  the  specifications  of  the  wood-pickling 
industry. 

From  the  "  anthracene  oils  "  there  is  obtained, 
by  suitable  treatment,  the  important  hydrocarbon 
anthracene,  which  is  used  as  the  starting  substance 
in  the  manufacture  of  alizarin,  and  of  other 
important  dyes. 

Although  the  amounts  of  the  different  compounds 
obtained  vary  with  the  nature  of  the  tar  and  the 
treatment  to  which  it  is  subjected,  the  following 
numbers  will  give  a  sufficiently  exact  idea  of  the 
relative  quantities  of  the  most  important  con- 
stituents yielded  by  one  ton  of  tar  : 

Benzene  and  toluene  .  25  Ibs. 

Phenol          .         .  .  ii    „ 

Cresols          .         .  50    „ 

Naphthalene          .  .  180    „ 

Creosote       .         .  .  200    „ 

Anthracene  .         .  .  6    ,, 

Benzene,  first  discovered  by  Faraday  in  1825,  is 
a  colourless,  mobile  liquid  which  boils  at  80-5°  C. 


THE  DISTILLATION  OF  COAL  TAR    21 

(176-9°  F.),  and  yields  a  readily  inflammable  vapour. 
Toluene  is  a  similar  compound  which  boils  at  111°  C. 
(231-8°  F.).  Phenol  or  carbolic  acid  is,  in  the  pure 
state,  a  white  crystalline  solid  which  melts  at  41°  C. 
(105-8°  F.).  Owing  to  the  very  large  amounts  of 
this  compound  used  in  the  manufacture  of  dyes, 
drugs,  and  explosives,  the  supply  obtained  from 
coal  tar  is  quite  insufficient,  under  present  con- 
ditions, to  meet  the  demand,  so  that  phenol  is  now 
manufactured  in  large  amount  from  benzene.  For 
this  purpose  benzene  is  first  treated  with  concen- 
trated sulphuric  acid,  and  the  resulting  product  then 
fused  with  caustic  soda.  Naphthalene  is  a  white 
crystalline  solid  which  melts  at  80°  C.  (176°  F.), 
and  anthracene  is  also  a  white  crystalline  solid 
which  melts  at  213°  C.  -(415°  F.). 


CHAPTER  III 

THE   CONSTITUENTS  OF  COAL  TAR  AND  THEIR 
APPLICATIONS  IN  THE  RAW  STATE 

IN  the  later  chapters  of  this  book  we  shall  discuss 
some  of  the  marvellous  transformations  which 
chemists  have  effected  in  the  constituents  of  coal 
tar,  transformations  which  are  the  basis  of  those 
great  chemical  industries  of  synthetic  dyes  and 
drugs  whose  development  during  the  past  half 
century  has  so  impressed  the  public  mind.  But  it 
must  not  be  forgotten  that  there  are  other  industries 
dependent  on  the  distillation  products  of  coal  tar ; 
industries  which  if  not,  like  the  chemical  ones, 
suffused  with  romance,  contribute  in  no  small 
measure  to  the  welfare  of  man  and  together  make  up 
a  large  part  of  the  wealth  derived  from  coal  tar. 
Indeed,  it  is  probably  to  these  industries  which 
depend  on  the  use  of  the  coal-tar  products  in  the 
raw  state  that  the  tar  distiller  mainly  looks  for  the 
maintenance  of  his  profits,  and  a  brief  account  of 
them  must  not  be  omitted  here. 

Benzol  and  Naphtha 

Although  it  is  as  a  raw  material  in  chemical 
industry,  in  the  manufacture  of  dyes,  drugs,  and 


THE  CONSTITUENTS  OF  COAL  TAR     23 

explosives,  that  pure  benzene  and  toluene  find 
their  chief  use,  large  quantities  of  the  various  grades 
of  commercial  benzol  are  now  employed  as  solvents 
in  the  preparation  of  paints  and  varnishes,  and  for 
other  purposes.  The  use  of  benzol  as  a  solvent 
goes  back,  indeed,  to  the  earliest  days  of  coal-tar 
distillation,  although  at  that  time  it  was  the  higher 
boiling  fractions  which  mainly  found  employment. 
At  the  present  day,  however,  the  industrial  applica- 
tions of  the  lower  boiling  fractions,  the  higher 
grades  of  commercial  benzol,  have  attained  a  great 
and  increasing  importance.  By  reason  of  its  solvent 
power,  benzol  is  largely  employed  as  a  detergent 
for  the  removal  of  grease,  wax,  and  paint  spots 
(for  which  purpose  it  is  frequently  mixed  with 
alcohol  and  ammonia),  and  as  a  solvent  for  gums 
and  resins  in  the  manufacture  of  varnishes  and 
lacs,  as  well  as  of  enamel,  bronze,  and  aluminium 
paints,  of  which  a  natural  gum  or  resin,  such  as 
Damar  gum,  forms  the  base.  Similarly,  by  reason 
of  its  solvent  power  for  resins,  benzol  is  used  in  the 
preparation  of  paints  used  in  painting  resinous 
woods,  the  partial  solution  of  the  resin  by  the  benzol 
affording  a  better  penetration  or  "  tooth  "  to  the 
paint.  Of  great  importance,  also,  is  the  use  of 
benzol  in  the  preparation  of  rubber  solutions  for 
use  as  cements  and  insulating  varnishes,  and  as  a 
solvent  for  sulphur  monochloride  in  the  cold  vulcan- 
isation of  rubber.  In  recent  times  benzol  has  been 


24        THE  TREASURES  OF  COAL  TAR 

used  in  large  amount,  more  especially  in  France 
and  Germany,  as  a  motor  fuel,  and  in  the  future  it 
will  doubtless  find,  in  this  direction,  a  vastly  more 
widespread  application.  This  cannot  but  exercise 
a  powerful  influence  on  the  whole  coal-tar  industry. 

The  higher  boiling  fractions  of  the  "  light  oil," 
the  naphthas,  find  their  chief  applications  as  solvents 
in  the  preparation  of  rubber  waterproof  material, 
and  as  illuminants  for  use  in  large  open  spaces ; 
and  the  flaring  light  of  the  naphtha  lamp  has  cast 
its  beams  for  many  years  now  over  the  wares  on 
the  costermonger's  barrow  and  the  stalls  and  booths 
of  the  open  market-place. 

In  1911  nearly  2,000,000  gallons  of  coal-tar 
solvents  were  produced  in  the  United  States,  and 
were  distributed  among  the  different  industries 
approximately  as  follows  (Weiss)  : 

Paint  and  varnish       .         .     47  per  cent. 
Rubber  and  rubber  cements     18 
Imitation  leathers       .         .10        ,, 
Chemical  manufactures        .     n        ,, 
Miscellaneous     .         .  14        ,, 

Similar   details   are,    unfortunately,    not   available 
with  regard  to  the  United  Kingdom. 

Carbolic  Acid  and  Naphthalene 

"  Crude  carbolic  acid/'  which  is  separated,  as 
we  have  already  seen,  from  the  "  middle  oils " 


THE  CONSTITUENTS  OF  COAL  TAR     25 

obtained  in  the  distillation  of  coal  tar,  consists  for 
the  most  part  of  various  "  tar  acids,"  more  especially 
carbolic  acid  and  cresylic  acid,  the  latter  being  a 
mixture  of  three  isomeric  compounds  (see  p.  44), 
known  as  cresols.  Although  the  pure  compounds, 
more  especially  pure  carbolic  acid  or  phenol  (to 
give  it  its  systematic  name),  are  used  to  a  large 
extent  in  chemical  industry,  they  also  find  a  very 
extensive  application  in  the  raw  state  as  antiseptics 
and  disinfectants.  As  such  the  cresols  are  more 
powerful,  and  at  the  same  time  less  poisonous,  than 
the  more  familiar  carbolic  acid  or  phenol. 

Owing  to  a  more  widespread  knowledge  of  the 
causes  of  disease  and  to  greater  efforts  made  in  the 
promotion  of  hygiene,  the  demand  for  antiseptics 
and  disinfectants  has  greatly  increased  during 
the  past  two  or  three  decades,  and  as  a  con- 
sequence we  now  find  on  the  market  very  many 
disinfectant  preparations  of  which  carbolic  and 
cresylic  acids  form  the  basis.  Since  these  acids 
are  not  very  soluble,  the  preparation  of  concen- 
trated disinfectants  which  would  mix  completely 
with  water  presented  some  difficulty ;  but  this 
difficulty  was  overcome  by  the  addition  of  a  certain 
amount  of  soft  soap  (whereby  the  tar  oils  present 
are  emulsified),  and  a  large  number  of  disinfecting 
fluids  are  now  prepared  on  this  general  principle. 
For  this  purpose  use  is  made  not  only  of  the  carbolic 
and  cresylic  acids  obtained  from  the  carbolic  oils, 


26        THE  TREASURES  OF  COAL  TAR 

but  also  of  the  lower  fractions  separated  from  the 
creosote  oils  which  are  specially  rich  in  cresols  and 
other  similar  compounds.  Thus  the  well-known 
liquid  lysol  consists  essentially  of  a  mixture  of 
cresols  (about  50  per  cent.)  with  a  potash  soap 
(about  20  per  cent.)  prepared  from  linseed  oil,  and 
a  certain  amount  of  glycerin.  The  cresols  also 
form  the  essential  constituent  of  Jeyes'  Fluid, 
Cresolin,  and  other  disinfectants. 

Phenol  and  cresol  may  also  be  incorporated  in 
ordinary  hard  soaps  or  mixed  with  various  other 
solid  materials ;  and  many  disinfectants  of  this 
nature,  more  or  less  efficient,  are  now  sold  under 
different  names. 

Naphthalene,  apart  from  its  important  uses  in 
chemical  industry,  is  now  employed  mainly  as  a 
disinfectant  and  as  a  preservative  against  the 
attack  of  moths  and  other  insects. 

Creosote 

That  tar  and  pitch  are  valuable  preservatives 
for  wood  has  long  been  known,  tar  having  been 
used  for  this  purpose  even  in  the  days  of  ancient 
Greek  civilisation.  But  it  is  only  in  comparatively 
recent  times  that  the  antiseptic  and  preservative 
properties  of  tar  have  been  applied  on  an  extensive 
scale.  Various  antiseptics,  such  as  corrosive  sub- 
limate and  copper  sulphate,  were  already  in  use 
for  the  preservation  of  timber  and  its  protection 


THE  CONSTITUENTS  OF  COAL  TAR      27 

against  the  attack  of  dry-rot  and  other  fungoid 
growths,  but  the  use  of  coal  tar  on  a  large  scale 
dates  only  from  the  introduction  of  the  timber- 
pickling  process  by  John  Bethell  in  1838.  This 
industry  soon  experienced  a  very  rapid  develop- 
ment owing  to  the  growth  more  especially  of  railway 
and  telegraph  systems  throughout  this  country 
and  the  world.  As  a  preservative  for  wood  which 
is  buried  in  the  ground  or  submerged  in  water, 
as  a  protective  even  against  the  formidable  Teredo 
navalis  and  other  marine  organisms,  coal-tar  creosote 
has  been  found  superior  to  all  other  materials. 

In  carrying  out  the  "  pickling  "  or  "  creosoting  " 
of  wood,  the  latter  is  placed  in  a  large  cylindrical 
boiler  and  the  air  is  then  very  thoroughly  exhausted 
by  means  of  a  pump.  In  this  way  the  air  is  with- 
drawn from  the  pores  of  the  wood.  Creosote, 
heated  to  a  temperature  of  about  100°  C.  (212°  F.), 
is  then  allowed  to  flow  into  the  boiler,  the  process 
of  exhaustion  being  still  maintained  for  some  time 
in  order  that,  at  the  higher  temperature,  the  moisture 
in  the  wood  may  also  be  withdrawn.  On  now 
admitting  air  into  the  boiler,  the  creosote  is  in- 
jected into  the  cells  of  the  timber,  and  the  process 
of  injection  is  completed  by  means  of  a  force  pump, 
the  pressure  within  the  boiler  being  raised  to  eight 
or  ten  atmospheres.  In  other  processes  the  timber  is 
impregnated  with  creosote  under  increased  pressure, 
and  then  maintained  for  some  time  under  greatly  re- 


28        THE  TREASURES  OF  COAL  TAR 

duced  pressure.    Under  this  treatment  a  cubic  foot 
of  wood  absorbs  about  one  gallon  of  creosote  oil. 

When  one  thinks  of  the  countless  rows  of  wooden 
sleepers  which  mark  out  the  railway  tracks  in  the 
different  countries  of  the  world,  of  the  never-ending 
lines  of  telegraph  poles  which  cany  their  network 
of  wires  across  whole  continents,  or  of  the  wooden 
piles  and  wharves  exposed  to  the  waters  of  every 
ocean,  one  will  understand,  in  some  measure,  how 
important  this  creosote  industry  has  become.  Since 
by  its  treatment  with  tar  oil  the  life  of  the  wood  is 
trebled  or  quadrupled,  it  will  readily  be  realised 
not  only  that  there  is  an  enormous  saving  effected 
in  the  cost  of  upkeep  of  railway  sleepers,  telegraph 
poles,  wooden  wharves,  etc.,  but  that  there  is  also 
a  consequent  great  reduction  in  the  consumption 
of  timber — a  matter  of  increasing  importance  in 
these  days  when  the  reserves  of  timber  throughout 
the  world  are  being  rapidly  depleted.  In  this 
country  upwards  of  50,000,000  gallons  of  creosote 
are  produced  annually,  and  most  of  this  is  used  for 
the  treatment  of  timber.  In  1913,  over  36,000,000 
gallons  of  creosote,  having  a  value  of  £592,000, 
were  exported,  mainly  to  the  United  States,  where, 
owing  to  the  enormous  extent  of  the  railway  system, 
the  demand  for  creosote  oil  is  much  greater  than 
the  supply.1  It  was  this  wood-pickling  industry 

1  "  In  1913  the  United  States  consumed,  for  timber  preserva- 
tion, over  90,000,000  gallons  of  creosote  oil,  and  of  this,  62  per 


THE  CONSTITUENTS  OF  COAL  TAR     29 

which  "  saved  the  situation  "  in  the  early  days  of 
coal-tar  production,  and  it  forms  at  the  present  day 
by  far  the  most  important  outlet  for  the  coal-tar  oils. 

The  antiseptic  and  disinfecting  properties  of 
creosote  oil,  which  are  not  entirely  due  to  the 
presence  of  carbolic,  cresylic,  and  other  tar  acids, 
have  also  led  to  the  extensive  use  of  creosote  in  the 
preparation  of  cattle  washes,  sheep  dips,  and  general 
disinfectants.  In  this  case  the  oil  is  generally 
mixed  with  a  quantity  of  soft  soap,  whereby,  owing 
to  the  emulsifying  action  of  the  soap,  a  very  fine 
emulsion  can  be  obtained  with  water. 

Creosote  oil,  suitably  fractionated  by  distillation, 
also  finds  application  as  a  liquid  fuel  for  internal 
combustion  (Diesel)  engines,  as  an  illuminant  (in 
the  "  Lucigen  "  lamp),  in  the  production  of  lamp- 
blafck,  for  softening  hard  pitch,  and  for  "  scrubbing  " 
coal  and  coke-oven  gas  for  the  recovery  of  benzene 
and  toluene. 

Refined  Tars  and  Pitch 

In  recent  years  owing  largely  to  improved  methods 
of  road  construction  and  to  the  desirability,  in  view 
of  the  great  increase  of  motor  traffic,  of  obtaining 
dust-free  roads,  an  increased  demand  for  refined 
tars  and  pitch  has  sprung  up.  For  some  time,  it 

cent,  was  imported  from  Europe.  Between  60  and  70  per  cent, 
of  the  total  quantity  of  oil  consumed  was  used  for  the  treatment 
of  railway  ties,  some  25,000,000  being  thus  treated  "  (E.  Stans- 
field  and  F.  E.  Carter  :  Report  to  Department  of  Mines,  Canada) . 


30        THE  TREASURES  OF  COAL  TAR 

is  true,  there  existed  considerable  prejudice  against 
the  practice  of  sprinkling  the  roads  with  tar  owing 
to  the  supposed  harmful  effects  of  the  tar  on  sur- 
rounding vegetation  and  the  irritating  action  of 
the  dust  from  such  roads  on  the  eyes.  But  the 
fear  of  harmful  effects  has  been  shown  to  be  with- 
out real  foundation,  and  the  tar-sprinkling  of  many 
of  our  main  thoroughfares  has  proved  a  great  boon. 
The  tar  used  for  this  purpose  must  be  fractionated 
so  as  to  satisfy  the  requirements  of  the  Road  Board, 
the  specification  of  which  lays  it  down  that :  "  The 
tar  shall  be  free  from  water,  and  on  distillation  shall 
yield  no  distillate  below  140°  C.  (284°  F.),  nor  more 
than  5  per  cent,  of  distillate  up  to  220°  C.  (428°  F.), 
which  distillate  shall  remain  clear  and  free  from 
solid  matter  (crystals  of  naphthalene,  etc.),  when 
maintained  at  a  temperature  of  30°  C.  (86°  F.),  for 
half  an  hour.  Between  140°  and  300°  C.  (284°  and 
572°  F.)  it  shall  yield  not  less  than  15  per  cent, 
nor  more  than  21  per  cent,  of  the  weight  of  the 
tar." 

The  various  grades,  also,  of  hard  and  soft  pitch, 
obtained  as  residues  from  the  distillation  of  coal 
tar,  have  found  valuable  applications  as  road- 
binding  material,  in  the  preparation  of  tar-mac, 
for  filling  the  joints  between  paving  stones,  for 
the  manufacture  of  roofing  felt,  for  making  coal 
briquettes,  for  electrical  insulation  and  for  other 
important  purposes. 


CHAPTER  IV 

MOLECULAR  ARCHITECTURE 

IN  the  preceding  chapters  we  have  seen  how  from 
the  black,  unsavoury  liquid,  coal  tar,  various  sub- 
stances have  been  isolated  and  have  found  import- 
ant applications  in  the  general  structure  of  our 
modern  civilisation.  But  the  benzene,  the  toluene, 
and  the  other  materials  to  which  reference  has 
already  been  made  exist  as  such  in  the  tar,  and 
their  separation  from  this  liquid  and  their  applica- 
tions, important  as  they  are,  are  not  such  as  to 
make  any  powerful  appeal  to  the  intellect  or  arouse 
a  feeling  of  wonder  in  the  mind.  Far  otherwise  is 
it,  however,  with  those  marvellous  transforma- 
tions which  have  been  brought  about  in  these  sub- 
stances by  chemists ;  transformations  which  have 
produced  from  some  eight  or  nine  colourless  liquids 
or  solids,  dyes  in  infinite  variety  which  rival  Nature's 
products  in  range  of  colour  and  delicacy  of  tone ; 
drugs  and  anaesthetics  which  purge  the  blood  of 
its  evil  humours  and  give  relief  from  pain ;  the 
sweet-smelling  essences  of  flowers ;  and  explosives 
which  give  power  and  strength  to  the  arm  of  man 
in  peace  as  well  as  in  war.  These  are  triumphs 

31 


32        THE  TREASURES  OF  COAL  TAR 

of  the  human  intellect  and  as  such  command  the 
admiration  and  wonder  of  thinking  men.  Not 
only  has  the  chemist  prepared  numberless  com- 
pounds hitherto  unknown,  but  he  has  entered  into 
competition  with  Nature  herself  and  has  success- 
fully broken  the  monopoly  which  heretofore  she 
had  enjoyed  in  the  production  of  many  important 
compounds.  So  successful,  indeed,  has  the  chemist 
been,  that  these  artificial  products  have,  in  some 
cases  at  least,  driven  the  natural  products  entirely 
out  of  the  market.  But  this  rivalry  with  Nature, 
the  task  of  building  up  or  synthesising  numerous 
highly  complex  compounds  from  the  simple  materials 
contained  in  coal  tar,  would  have  been  hopeless 
without  the  aid  of  some  guiding  principle.  It  is 
necessary,  therefore,  before  passing  to  the  discussion 
of  the  substances  which  have  been  evolved  by 
chemists  from  the  constituents  of  coal  tar,  to  give 
a  short  account  of  the  theory  of  molecular  structure 
by  which  chemists  have  directed  their  labours. 
The  understanding  of  the  processes  by  which  these 
compounds  are  produced  will  thereby  be  facilitated. 

For  convenience  in  representing  chemical  elements 
and  compounds,  the  Swedish  chemist  Berzelius 
introduced,  a  century  ago,  a  system  of  symbols, 
each  of  which  consists  of  one  or  two  letters  and 
represents  one  atom  of  the  particular  element. 
Thus,  C,  H,  O,  N,  for  example,  represent  one  atom 


MOLECULAR  ARCHITECTURE  33 

or  smallest  particle  of  the  elements  carbon,  hydro- 
gen, oxygen,  and  nitrogen  respectively.  But  a  com- 
pound can  be  regarded  as  being  formed  by  the 
combination  or  uniting  of  the  atoms  of  the  con- 
stituent elements  in  certain  definite  proportions, 
,  and  so  we  can  conveniently  represent  the  molecule, 
or  smallest  particle  of  a  compound,  by  writing  the 
symbols  of  the  constituent  elements  side  by  side. 
Thus,  CO  represents  a  compound  of  carbon  and 
oxygen,  the  molecule  of  which  contains  one  atom 
of  carbon  and  one  atom  of  oxygen ;  and  NO, 
similarly,  represents  a  compound  of  nitrogen  and 
oxygen.  Frequently,  however,  the  molecule  of  a 
compound  is  formed  by  the  combination  of  elements 
in  more  than  one  atomic  proportion,  and  so  we 
write,  for  example,  H2O,  which  is  the  formula,  as 
it  is  called,  for  water.  This  formula  indicates  that 
the  molecule  of  water  contains  two  atoms  of  hydro- 
gen and  one  atom  of  oxygen.  The  formula  NH3, 
similarly,  which  is  the  formula  for  ammonia,  indi- 
cates that  the  molecule  of  this  compound  contains 
three  atoms  of  hydrogen  united  with  one  atom  of 
nitrogen. 

It  might,  perhaps,  be  thought  that  an  infinite 
number  of  compounds  could  be  formed  by  the 
combination  of  the  atoms  of  two  elements  in  differ- 
ent proportions,  e.g.  HO,  H2O,  H3O,  etc.  But 
although  no  a  priori  reason  can  be  given  against 
the  possibility,  it  has  been  found  that,  as  a  matter 


34        THE  TREASURES  OF  COAL  TAR 

of  fact,  elementary  atoms  do  not  possess  this  un- 
limited power  of  combination  ;  and  the  recognition 
of  this  important  fact  is  embodied  in  the  doctrine 
of  valency,  a  doctrine  which  we  owe  to  the  late  Sir 
Edward  Frankland.  As  no  element  is  known  which 
has  a  lower  combining  power  than  hydrogen,  this 
element  is  taken  as  the  standard  of  reference  and  is 
said  to  have  unit  combining  power  or  unit  valency, 
or  to  be  univalent.  Oxygen,  one  atom  of  which 
can  combine  with  two  atoms  of  hydrogen  (as  in 
water,  H2O),  is  said  to  be  bivalent,  and  carbon, 
one  atom  of  which  can  combine  with  four  atoms 
of  hydrogen,  is  said  to  be  quadrivalent.  Since  an 
atom  of  carbon  is  never  found  to  combine  with  more 
than  four  atoms  of  hydrogen,  the  carbon  is,  in  this 
case,  said  to  be  saturated  ;  and  the  compound 
CH4,  which  represents  methane  or  marsh  gas,  is 
spoken  of  as  a  saturated  hydrocarbon. 

Although  there  is,  of  course,  no  material  link  or 
bond  between  the  atoms,  we  can,  nevertheless,  re- 
present union  between  atoms  as  if  it  were  material, 
by  means  of  a  line  or  lines,  according  to  the  valency 
of  the  atom.  Thus  we  can  represent  the  molecule 
of  methane  by  the  diagrammatic  or  graphic  formula, 

H 
H—  C—  H 


But  the  element  carbon  is  remarkable  among  all  the 


MOLECULAR  ARCHITECTURE  35 

elements  in  its  property  of  combining  also  with  other 
atoms  of  carbon  and  so  forming  "  chains  "  of  carbon 
atoms  ;  and  we  therefore  obtain  a  series  of  com- 
pounds which  may  be  represented  by  the  diagrams  : 

H  H  H 

H—  C—  C—  C—  H     ;     etc. 

Ill 
H    H    H 

C3H8  (Propane) 

This  series  of  hydrocarbons  is  generally  known 
as  the  methane  series,  and  to  it  belong  gasoline, 
petrol,  vaseline,  and  paraffin  wax. 

There  are  also  other  hydrocarbons  which  contain 
a  lower  proportion  of  hydiogen  and  are  therefore 
said  to  be  unsaturated.  Thus,  if  we  take  away  two 
hydrogen  atoms  from  each  of  the  compounds  of 
the  methane  series,  we  obtain  hydrocarbons  which 
can  be  represented  by  the  formulae  : 

H  H 

H\  /H         H\          |      | 

>C  =  C<          ;          >C  =  C—  C—  H     ;    etc. 
H/  \H         H/  | 

H 


(Ethylene)  C3H6  (Propylene) 

These  constitute  another  series  of  hydrocarbons 
known  by  the  name  of  the  first  member,  ethylene. 

These  diagrammatic  formulae,  it  should  be  em- 
phasised, are  not  intended  to  represent  the  spatial 
arrangement  of  the  atoms  ;  there  is,  indeed,  reason 
to  believe  that  these  "  chains  "  of  carbon  atoms 
would  form  a  spiral  in  space.  These  formulae, 


36        THE  TREASURES  OF  COAL  TAR 

rather,  are  intended  merely  to  indicate  that  in  the 
molecule  of  a  compound  the  constituent  atoms  are 
not  present  in  disordered  array  but  are  associated 
in  some  definite  manner,  certain  atoms  being 
attached,  as  it  were,  to  certain  other  atoms,  although 
by  no  material  bond  or  connection,  just  as  a  satellite 
may  be  said  to  be  attached  to  a  planet.  We  can, 
therefore,  also  write  the  above  formulae  in  a  some- 
what more  compact  form,  and  represent  propane, 
for  example,  by  the  formula  CH3-CH2-CH3  (the 
"  bond "  between  the  carbon  atoms  being  now 
represented  by  a  dot),  and  propylene  by 

CH2 :  CH-CH3. 

This  theory  of  chemical  structure,  which  depends, 
as  we  see,  on  the  recognition  of  the  quadrivalency  of 
the  carbon  atom,  is  due  to  August  von  Kekule,  and 
was  put  forward  by  him  in  1858.  The  origin  of 
the  theory  has  been  recounted  by  Kekule  himself. 
During  a  period  of  residence  in  London  he  was 
returning  from  a  visit  paid  at  Islington  to  where 
he  stayed  at  Clapham.  "  One  fine  summer  even- 
ing," he  relates,  "  I  was  returning  by  the  last 
omnibus,  '  outside  '  as  usual,  through  the  deserted 
streets  of  the  metropolis,  which  are  at  other  times 
so  full  of  life.  I  fell  into  a  reverie,  and  lo  !  the 
atoms  were  gambolling  before  my  eyes  !  When- 
ever, hitherto,  these  diminutive  beings  had  appeared 
to  me,  they  had  always  been  in  motion ;  but  up 


MOLECULAR  ARCHITECTURE  37 

to  that  time  I  had  never  been  able  to  discern  the 
nature  of  their  motion.  Now,  however,  I  saw  how, 
frequently,  two  smaller  atoms  united  to  form  a 
pair  ;  how  a  larger  one  embraced  two  smaller  ones  ; 
how  still  larger  ones  kept  hold  of  three  or  even  four 
of  the  smaller ;  whilst  the  whole  kept  whirling  in 
a  giddy  dance.  I  saw  how  the  larger  ones  formed 
a  chain,"  .  .  .  And  then  he  adds  :  "I  spent  part 
of  the  night  in  putting  on  paper  at  least  sketches 
of  these  dream-forms."  From  these  sketches  were 
developed  the  structural  formulae  of  which  ex- 
amples have  just  been  given. 

The  saturated  hydrocarbons,  methane,  ethane, 
etc.,  may  be  regarded  as  the  parents  of  a  countless 
brood  of  other  compounds  derived  from  them  by 
the  substitution  or  replacement,  direct  or  indirect, 
of  one  or  more  hydrogen  atoms  by  the  atoms  of 
other  elements  or  by  groups  of  elements  ("  radicles  ") 
which  pass  from  compound  to  compound  like  single 
atoms.  Thus,  by  substituting  one  atom  of  hydrogen 
in  the  saturated  hydrocarbons  by  an  atom  of  iodine, 
we  get  a  series  of  iodides,  CH3-I,  C2H5-I,  C3H7-I, 
etc.  ;  or  by  substituting  one  atom  of  hydrogen 
by  the  group  OH  (hydroxyl),  we  obtain  a  series 
of  compounds  known  as  alcohols  ("  alcohol "  in 
chemistry  is  a  generic  name),  thus,  CH3-OH,  methyl 
alcohol  or  "  wood-spirit  "  ;  C2H5-OH,  ethyl  alcohol 
or  "  spirits  of  wine  "  ;  and  so  on,  the  groups  of 
atoms  CH3,  C2H5,  being  known  as  methyl  and  ethyl. 


38        THE  TREASURES  OF  COAL  TAR 

It  will  readily  be  understood  from  this  that  the 
possible  number  of  compounds  is  exceedingly  large, 
and,  for  this  reason,  the  study  of  the  compounds 
of  carbon — the  number  of  which  at  the  present 
day  exceeds  150,000 — has  developed  into  a  special 
branch  of  chemistry  known  as  organic  chemistry. 

Since  the  hydrocarbons  of  the  methane  series 
are  formed  of  "  chains  "  of  carbon  atoms,  so  also 
are  the  compounds  derived  from  them ;  and  since 
the  natural  animal  and  vegetable  fats  and  oils  are 
amongst  these  compounds,  the  term  "  fatty "  or 
"  aliphatic  "  (&Xet<pa,p  =  fat)  has  been  applied  to 
the  whole  group  or  class  of  compounds. 

In  studying  the  carbon  compounds  we  meet  with 
a  phenomenon  which,  although  not  unknown  in  the 
compounds  of  the  other  elements,  is  found  with 
extraordinary  frequency  amongst  the  former.  This 
is  the  phenomenon  to  which  the  name  of  isomerism 
has  been  given. 

One  of  the  fundamental  laws  of  chemistry  states 
that  the  composition  of  a  compound — that  is,  the 
nature  and  number  of  the  atoms  present  in  the 
molecule — is  constant  and  definite  (Law  of  Constant 
Proportions),  and  for  long  it  was  believed  that 
the  converse  statement  also  was  true,  namely, 
that  only  a  single  compound  could  exist  correspond- 
ing with  a  particular  composition.  As  the  number 
of  compounds  became  multiplied,  it  began  to  be 


MOLECULAR  ARCHITECTURE  39 

observed  more  and  more  frequently  that  the  same 
elements  might  be  united  in  the  same  proportions 
and  yet  yield  compounds  with  entirely  different 
properties.  It  is  to  this  phenomenon  that  the  term 
isomerism  is  applied.  Just  as  the  same  set  of  bricks 
can,  by  varying  their  arrangement,  be  formed  into 
structures  of  totally  different  kinds,  so  also  the  same 
atoms  can,  by  varying  their  arrangement  within 
the  molecule,  give  rise  to  different  atomic  structures, 
or  different  compounds.  We  are  led,  therefore, 
to  the  recognition  of  the  fact  that  the  properties  of 
a  compound  depend  not  merely  on  its  composition, 
but  also  on  its  internal  structure,  or  the  arrange- 
ment of  the  atoms  within  the  molecule.  A  know- 
ledge of  this  atomic  arrangement  or  constitution  of 
the  molecule  is  of  the  highest  importance,  and  is, 
indeed,  essential  for  the  successful  building  up  or 
synthesis  of  a  compound  from  simpler  materials, 
such  as  we  shall  discuss  in  the  following  chapters. 
It  is  because  the  theory  of  Kekule  enables  one  to 
represent  molecular  constitution  and  to  foresee  the' 
possible  existence  of  isomeric  compounds  that  it 
has  exercised  such  an  important  influence  on  the 
development  of  organic  chemistry.  Thus,  if  we 
have  the  compound  CH3-CH2-CH3,  it  is  clear  that 
we  can  replace  one  atom  of  hydrogen  in  this  com- 
pound by  an  atom,  say,  of  iodine,  in  two  ways, 
so  that  we  should  obtain  either  the  compound 
CH3-CH2-CH2I,  or  the  compound  CH3-CHI-CH3, 


40        THE  TREASURES  OF  COAL  TAR 

the  iodine  being  attached,  in  the  former  case,  to 
a  terminal  carbon  atom,  and  in  the  latter  case  to 
the  intermediate  carbon  atom.  Accordingly,  there 
should  exist  two  and  only  two  different  compounds 
having  the  composition  C3H7I ;  and  as  a  matter  of 
fact  two  compounds  and  only  two  are  known. 

Although  a  number  of  hydrocarbons  belonging 
to  the  methane  series  are  found  in  the  tar  which 
is  produced  by  distilling  coal  at  a  low  temperature, 
they  are  not  met  with  in  the  ordinary  gas  or  coke- 
oven  tar,  and  they  are  of  only  secondary  importance 
in  the  manufacture  of  coal-tar  dyes  and  other  pro- 
ducts. The  most  important  compounds  occurring 
in  gas  and  coke-oven  tar  are,  as  we  have  seen, 
benzene,  toluene,  xylene,  phenol,  cresol,  naphthalene, 
and  anthracene,  these  being  the  compounds  from 
which,  for  the  most  part,  the  endless  array  of  dyes 
and  other  coal-tar  products  has  been  derived. 
These  compounds,  however,  belong  to  quite  a 
different  class  from  those  already  described ;  they 
possess  a  totally  different  constitution,  and  belong, 
as  it  were,  to  a  different  type  of  molecular 
architecture.  From  the  fact  that  many  of  the 
compounds  which  occur  naturally  and  belong  to 
this  group  possess  a  distinct  odour  or  "  aroma," 
the  term  "  aromatic  "  has  been  given  to  the  com- 
pounds belonging  to  this  division  of  organic 
chemistry. 


MOLECULAR  ARCHITECTURE  41 

Just  as  we  have  seen  that  methane  may  be  re- 
garded as  the  first  parent  of  the  compounds  be- 
longing to  the  aliphatic  group,  so  benzene  (C6H6) 
may  be  called  the  parent  of  the  aromatic  compounds  ; 
and  it  is  to  Kekule  also  that  we  owe  the  elucidation 
of  the  structure  of  this  important  hydrocarbon. 
Again  Kekule  had  a  dream.  He  was  now  (1865) 
in  Ghent  and  dozed  before  the  fire.  Again  he  saw 
the  atoms  gambolling  before  his  eyes,  the  chains 
twining  and  twisting  in  snake-like  motion.  "  But 
look  !  What  was  that  ?  One  of  the  snakes  had 
seized  hold  of  its  own  tail,  and  the  form  whirled 
mockingly  before  my  eyes.  As  if  by  a  flash  of 
lightning  I  awoke  "  ;  .  .  .  but  the  picture  Kekule 
had  seen  of  the  snake  which  had  seized  hold  of  its 
own  tail  gave  him  the  clue  to  one  of  the  most  puzzling 
molecular  structures,  the  structure  of  the  benzene 
molecule,  a  ring  of  six  carbon  atoms  to  each  of  which 
a  hydrogen  atom  is  attached.  Thus  we  obtain  the 
structural  formula  of  the  benzene  molecule  : 


H 

HC         C-H 

II 
C-H 


HC 


H 

the  "  ring  "  of  carbon  atoms  being  written  in  the 
form  of  a  hexagon  instead  of  in  the  form  of  a  circle. 
Since  this  structure  occurs  very  frequently  in  the 


42        THE  TREASURES  OF  COAL  TAR 

formulae  of  coal-tar  products,  it  is  generally  simpli- 
fied to  the  skeleton  form  by  omitting  the  symbols 
for  carbon  and  hydrogen.  We  thus  obtain  as  the 
diagrammatic  representation  of  the  benzene  mole- 
cule the  simple  hexagon : 


From  methane,  as  we  saw,  many  other  compounds 
could  be  derived  by  replacing  one  or  more  atoms  of 
hydrogen  by  the  atoms  of  other  elements  and  by 
radicles  or  groups  of  elements.  So  also  from  benzene 
whole  series  of  compounds  can  be  similarly  derived. 
Thus,  if  we  replace  one  atom  of  hydrogen  by  the 
group  or  radicle  CH3  (methyl),  we  obtain  the  hydro- 
carbon toluene,  the  formula  of  which,  C6H5-CH3, 
will  be  represented  by  the  diagram  : 


CH3 


Similarly  phenol  or  carbolic  acid  is  derived  from 
benzene  by  the  replacement  of  one  atom  of  hydro- 
gen by  the  group  OH  (hydroxyl),  and  so  we  obtain 


the  formula  C6H5-OH  or 


•OH 


MOLECULAR  ARCHITECTURE 


43 


In  the  case  of  naphthalene,  which  is  a  hydrocarbon 
having  the  formula  C10H8,  we  have  two  benzene 
"  rings  "  joined  together  thus  : 


H        H 

^X/6 
H-C         C 


\H 


or,  more  simply, 
H-C         C         C-H 


H         H 


whereas,  in  the  case  of  anthracene,  C14H10,  we  have 
three  "  rings  "  : 


H         H         H 


x^CN 
H-C         C 


H 


H 


\H 
C-H 


H 


In  the  case  of  the  aliphatic  compounds  we  saw  how, 
according  to  the  theory  of  Kekule,  the  existence  of 
isomeric  compounds  could  be  foreseen  and  explained. 
In  the  case  of  the  compounds  derived  from  benzene, 
similarly,  isomerism  can  occur,  but  this  isomerism 
is  found  only  when  more  than  one  atom  of  hydrogen 
is  substituted  or  replaced.  Thus,  for  example, 
when  one  atom  of  hydrogen  is  substituted  by  the 
methyl  group,  CH3,  and  another  by  the  hydroxyl 


44        THE  TREASURES  OF  COAL  TAR 

group,  OH,  we  can  obtain  three  different  arrange- 
ments, represented  by  the  formulas : 

CH3 


'OH 

OH 

and  these  three  different  arrangements  correspond 
with  three  distinct  isomeric  compounds  known 
respectively  as  ortho-cresol,  meta-cresol,  and  para- 
cresol,  the  terms  ortho-,  meta-,  and  para-  referring 
to  the  relative  positions  of  the  two  substituting 
groups.  These  three  isomeric  cresols  occur,  as  we 
have  seen,  in  coal  tar,  and  have  powerful  antiseptic 
properties.  Ortho-cresol  melts  at  31°  C.  (87-8°  F.), 
meta-cresol  at  5°  C.  (41°  F.),  and  para-cresol  at 
36°  C.  (96-8°  F.). 

It  may  seem,  perhaps,  to  some  that  whatever 
psychological  or  speculative  interest  the  dreams 
and  theories  of  Kekule  might  possess,  they  could 
have  no  importance  for  the  practical  life  of  the 
people.  And  yet  it  is  just  these  theories  which 
form  the  very  basis  and  fundament  of  those  great 
chemical  industries  which  command  the  wonder 
and  respect  of  all.  For  the  advance  and  develop- 
ment of  organic  chemistry,-— described  by  Wohler  as 
a  "  tropical  forest  primeval,  full  of  the  strangest 
growths,  an  endless  and  pathless  thicket,  in  which 


MOLECULAR  ARCHITECTURE  45 

a  man  may  well  dread  to  wander," — the  theories 
of  molecular  structure  were  as  important  as  is  a 
map  to  a  traveller  in  an  unknown  land.  Without 
them  we  could  not  have  witnessed  what  is,  perhaps, 
the  crowning  achievement  of  organic  chemistry, 
the  synthesis  of  many  of  Nature's  own  products 
as  well  as  of  the  innumerable  dyes,  therapeutic 
agents,  and  other  materials  which  are  regarded  as 
necessaries  in  our  modern  civilisation  and  in  respect 
of  which  this  country  is  now  endeavouring  to  make 
herself  independent  of  outside  supplies.  It  was, 
indeed,  largely  if  not  mainly  owing  to  her  neglect 
of  pure  science  and  of  scientific  theory,  and  owing 
to  the  fact  that  "  the  English  manufacturer  has 
considered  that  a  knowledge  of  the  benzol  market 
was  of  greater  importance  than  a  knowledge  of  the 
benzol  theory,"  that  this  country  lost  the  pre- 
eminence in  the  coal-tar  industry  which  in  the  early 
days,  when  that  industry  was  controlled  by 
chemists  (like  Perkin  and  Nicholson),  she  so  fully 
enjoyed. 

If  there  are  any  among  the  readers  of  this  book 
who  feel  some  dismay  at  the  aspect  of  the  formulae 
which  have  been  introduced  in  this  chapter  and  which 
will  be  employed  more  frequently  in  the  sequel, 
I  would  ask  them  to  believe  that  if  they  will  but 
make  the  slightest  effort  they  will  find  nothing  of 
which  to  be  afraid,  especially  if  they  will  bear  in 


46        THE  TREASURES  OF  COAL  TAR 

mind  that  these  formulas  need  not  be  memorised. 
No  reader  of  ordinary  intelligence  will  refrain  from 
reading  a  work  on  architecture  because  of  the 
drawings  of  pillar  capitals,  or  of  arches,  or  of  the 
plans  of  buildings  which  accompany  the  text.  On 
the  contrary,  without  these  drawings,  how  could  the 
reader  form  a  true  mental  picture  of  even  a  simple 
structure  or  understand  the  mutual  relationships 
of  its  parts  ?  So  is  it  with  the  formulae  which  we 
shall  employ  here.  These  formulae  are  the  plans, 
so  to  say,  of  molecular  structures  by  which  the  read- 
ing and  understanding  of  the  text  may  be  made 
more  easy.  By  looking  at  these  diagrams,  these 
formulae,  one  sees  at  a  glance  how  the  different 
compounds  are  related ;  how,  for  example,  from 
benzene,  the  parent  hydrocarbon,  there  have  been 
evolved  numerous  other  compounds  of  much  more 
complex  structure.  We  may,  indeed,  regard  the 
hexagon,  the  diagram  which  we  have  used  to  re- 
present the  structure  of  the  benzene  molecule,  as 
representing,  so  to  say,  a  simple  house  to  which 
succeeding  owners  may  add  at  their  pleasure — one 
a  bow  window,  another  a  turret,  a  third  an  additional 
room,  and  so  on — so  that  the  original  building  be- 
comes completely  transformed.  By  means  of  our 
formulae  we  shall  be  able  readily  to  follow  the  suc- 
cessive changes  which  take  place.  The  contem- 
plation of  these  formulae,  moreover,  has  a  value 
for  the  layman  in  that  he  will  thus  gain  some  idea 


MOLECULAR  ARCHITECTURE          47 

of  the  complexity  of  the  compounds  and  may  come 
to  appreciate  more  fully  the  ability  of  the  chemists 
who  have  not  only  succeeded  in  unravelling  the 
intricate  details  of  molecular  constitution,  but  have 
also  built  up  these  complex  structures  from  more 
simple  materials.  The  ordinary  person  is  impressed 
by  the  grandeur  or  magnitude  of  some  engineering 
triumph  and  may  be  overwhelmed  by  statistics 
of  the  number  of  bolts  or  nuts,  or  the  weight  of 
metal  employed  ;  but  although  the  intricate  struc- 
ture of  the  molecule  cannot  be  seen  by  the  eye, 
it  must  nevertheless  impress  the  mind  of  every 
thoughtful  person.  Indeed,  until  the  imagination 
of  our  people  can  be  fired  by  the  mental  contem- 
plation of  these  great  chemical  achievements  we 
shall  never  be  able  to  gain  for  chemistry  and  for 
chemical  study  that  measure  of  interest,  respect, 
recognition,  and  encouragement  which  alone  will 
enable  this  country  to  hold  her  own  in  the  industrial 
competition  of  the  world. 


CHAPTER  V 

THE  PRODUCTION  OF  DYES  FROM  COAL  TAR 

FROM  time  immemorial,  men,  denied  by  nature  the 
more  varied  and  gorgeous  colourings  of  the  animals, 
have  delighted  in  staining  their  bodies  or  dyeing 
their  garments  by  means  of  the  various  colouring 
matters  with  which  the  animal  and  vegetable 
creation  supplied  them — the  colouring  matter  of 
logwood ;  the  animal  dye  carmine  or  cochineal 
which  was  used  in  this  country  to  dye  the  scarlet 
tunics  of  our  soldiers  ;  the  blue  dye,  indigo  or  woad, 
one  of  the  oldest  of  dyes ;  the .  red  dye,  alizarin, 
obtained  from  the  root  of  the  madder-  plant  and 
employed  in  the  production  of  Turkey  red ;  and 
the  costliest  and  most  famous  dye  of  the  ancient 
world,  Tyrian  purple,  obtained  from  a  shell-fish 
found  on  the  eastern  shores  of  the  Mediterranean. 
Until  the  middle  of  the  nineteenth  century  these 
and  some  other  dyes,  mainly  of  animal  or  vegetable 
origin,  were  practically  the  only  dyes  with  which 
man  was  acquainted.  But  in  1856  a  new  chapter, 
and  one  of  the  highest  importance  in  the  history 
of  dyes,  commenced  with  the  discovery  of  the  once 
favourite  synthetic  dye,  mauve,  which  found  its  last 

48 


PRODUCTION  OF  DYES  FROM  COAL  TAR  49 

use  for  colouring  the  postage  stamps  of  the  late 
Victorian  era.  This  dye  was  prepared  from  crude 
aniline,  which  was  in  turn  produced  from  the  benzole 
derived  from  coal  tar,  and  it  was  the  first  of  a  long 
list  of  synthetic  dyes  prepared  by  chemists  from 
the  constituents  of  coal  tar.  Starting  from  benzene, 
toluene,  phenol,  naphthalene,  anthracene,  and  a 
few  other  constituents  of  the  thick  black  liquid, 
coal  tar,  which  less  than  a  hundred  years  ago  was 
a  useless  waste  material  and  a  nuisance  to  the  gas 
manufacturer,  synthetic  dyes,  to  the  estimated 
value  of  nearly  £20,000,000,  are  now  manufactured 
annually,  more  than  two- thirds  of  this  amount  being 
produced,  in  1913,  in  Germany.  These  dyes  have, 
by  reason  of  their  almost  infinite  variety  and 
applicability,  their  range  of  colour  and  delicacy 
of  tone,  ousted  the  natural  dyes  to  a  very  large 
extent  from  the  dye-works. 

It  is,  of  course,  now  universally  known  that  these 
numerous  coal-tar  dyes  are  not  present  as  such  in 
the  coal  tar,  but  that  they  are  obtained  from  the 
constituents  of  coal  tar  by  a  more  or  less  complex 
series  of  chemical  reactions.  Thus,  from  some  eight 
or  nine  primary  constituents  of  coal  tar  (benzene, 
toluene,  xylene,  phenol,  cresol,  naphthalene,  anthra- 
cene, etc.),  there  are  produced,  by  the  action  of 
various  chemical  reagents — nitric  acid,  sulphuric 
acid,  chlorine,  caustic  soda,  etc. — some  two  hundred 
and  ninety  "  intermediate  "  compounds,  and  from 


50        THE  TREASURES  OF  COAL  TAR 

these  "  intermediates/'  by  their  mutual  combina- 
tions and  interactions,  the  finished  dyes  are  pre- 
pared, of  which  upwards  of  nine  hundred  actually 
find  application  at  the  present  time.  The  production 
of  a  dye,  therefore,  is  by  no  means  a  simple  operation, 
except  in  a  very  few  cases,  and  is,  in  most  cases,  a 
very  complex  process  involving,  it  may  be,  fifteen 
or  twenty  distinct  and  separate  chemical  reactions. 
That  the  industry  of  dye-manufacture  is  a  very 
intricate  one  will,  therefore,  be  readily  understood, 
and  if  success  is  to  be  attained,  each  step  in  the 
process  of  manufacture  must  be  scientifically  con- 
trolled and  carried  out  with  the  highest  degree  of 
efficiency.  But  a  further  very  serious  complication 
is  introduced  owing  to  the  fact  that  in  the  prepara- 
tion of  many  of  the  intermediates,  "  by-products  " 
are  produced  in  varying  amounts,  and  for  these  by- 
products a  remunerative  outlet  must  be  obtained. 
Even  when  all  the  products  formed  in  the  manu- 
facture of  a  given  intermediate  can  be  used  up  in 
the  manufacture  of  dye-stuffs,  the  demand  for  the 
dye-stuffs  thus  obtained  may  differ  very  greatly, 
and  by  no  means  always  in  the  same  direction  or 
in  the  same  measure  as  the  intermediates.  The 
problem  of  working  up,  completely  and  remuner- 
atively, without  waste  and  without  over-production, 
all  the  by-products  formed  in  the  manufacture  of 
the  intermediates,  is  one  of  the  utmost  importance 
for  the  success  of  the  industry.  Owing  to  the 


PRODUCTION  OF  DYES  FROM  COAL  TAR  51 

enormous  development  of  her  organic  chemical 
industry,  embracing  the  manufacture  of  dyes,  drugs, 
perfumes,  and  "  fine  "  (organic)  chemicals  generally, 
the  solution  of  this  problem  has  become  more  easy 
for  Geimany  than  for  any  other  country. 

Although  Great  Britain  is  one  of  the  largest 
producers  of  coal  tar,  she  has,  hitherto,  manu- 
factured only  a  small  number  of  the  intermediates 
required  for  the  production  of  the  finished  dyes, 
and  has  contented  herself  with  importing  many  of 
the  most  important  intermediates  from  Germany. 
If,  therefore,  this  country  is  to  gain  her  independ- 
ence in  respect  of  the  manufacture  and  supply  of 
dyes,  she  must  undertake,  in  the  future,  the  pro- 
duction of  the  necessary  intermediates  on  a  greatly 
more  extensive  scale  than  in  the  past.  In  this 
direction  very  marked  advance  has  been  made  since 
the  outbreak  of  the  war. 

The  value,  in  round  figures,  of  the  estimated 
production  of  coal-tar  dyes  in  1912  is  given  in  the 
following  table  : 

Germany  ....  £13,500,000 
Switzerland  ....  1,290,000 
Great  Britain  .  .  .  1,190,000 
France  ....  1,000,000 

United  States  .  .  .  750,000 
Other  countries  .  .  .  2,000,000 


52        THE  TREASURES  OF  COAL  TAR 

In  1913  Great  Britain  imported  dyes  to  the  value 
of  £1,946,224,  the  value  of  the  dyes  obtained  from 
Germany  amounting  to  about  £1,800,000.  Of  all 
the  dyes  used  in  this  country  in  the  textile,  fur, 
feather,  paint,  and  other  industries,  only  about 
10  per  cent,  were  of  home  manufacture. 

In  the  year  1845,  largely  owing  to  the  efforts  of 
the  Prince  Consort  and  of  the  Queen's  physician, 
Sir  James  Clark,  there  was  founded  the  Royal 
College  of  Chemistry  in  London,  and  A.  W.  Hofmann, 
a  young  German  chemist  who  had  been  trained 
under  the  renowned  Justus  von  Liebig  at  Giessen, 
was  appointed  Professor  of  Chemistry.  For  some 
time  chemists  had  been  interesting  themselves  in 
the  nature  and  composition  of  coal  tar,  and  Hofmann 
and  his  students  engaged  energetically  in  the  work 
of  investigation.  One  of  the  earliest  results  to  be 
obtained  was  the  isolation  from  coal  tar  of  the  hydro- 
carbon benzene,  a  compound  which  was  first  dis- 
covered by  Michael  Faraday  in  1825  x ;  and  after 
the  work  of  Mansfield  (p.  15),  coal  tar  became  the 
chief  source  of  the  compound.  As  early  as  1834 
Mitscherlich  had  shown  that  when  benzene  is  treated 


1  In  1815  oil-gas  was  introduced  as  an  illuminant,  and  was 
supplied  to  the  consumers  in  cylinders  into  which  the  gas  had 
been  pumped  under  a  pressure  of  thirty  atmospheres.  Under 
this  pressure  a  portion  of  the  gas  condensed  to  a  liquid,  and  from 
this  liquid  Faraday  isolated  benzene,  or  bi-carburet  of  hydrogen 
as  he  called  it. 


PRODUCTION  OF  DYES  FROM  COAL  TAR  53 
with  concentrated  nitric  acid,  it  is  converted  into 

* 

an  oily  liquid,  nitro-benzene,  C6H5-NO2,  which, 
even  before  the  introduction  of  the  coal-tar  dyes, 
was  manufactured  in  small  quantity  and  used, 
under  the  name  of  Essence  of  Mirbane,  for  scenting 
soap.  Nitrobenzene,  in  its  turn,  as  was  found  by 
Bechamp  in  1854,  could  be  converted  into  aniline,1 
C6H5«NH2,  by  acting  on  it  with  a  mixture  of  acetic 
acid  and  finely  divided  iron.  We  see,  then,  that 
coal  tar  became  not  only  a  convenient  source  of 
supply  of  benzene,  but  also,  through  the  chemical 
transformation  of  this  substance,  of  the  compound 
aniline.  Entering  into  this  heritage  of  knowledge, 
W.  H.  Perkin,  who  had,  as  one  of  Hermann's  students, 
been  trained  in  an  atmosphere  of  purely  scientific 
investigation,  made  his  important  discovery  of  the 
first  coal-tar  dye.  It  was  in  1856,  while  engaged 
in  an  attempt  to  produce  the  naturally-occurring 
alkaloid  quinine  from  simpler  substances,  that 
Perkin  treated  a  solution  of  aniline  in  dilute  sul- 
phuric acid  with  potassium  dichromate.  As  a 
result,  there  separated  out  from  the  liquid  a  dark- 
coloured,  resinous  mass,  and  from  this  unpromising 
material  Perkin  separated  the  first-known  aniline 
dye,  which  he  somewhat  later  manufactured  and 
sold  under  the  name  of  "  aniline  purple,"  or  "  Tyrian 
Purple,"  or  "  mauve,"  the  name  given  to  it  by  the 

1  Derived  from  anil,  the  Portuguese  name  for  indigo,  from 
which  aniline  was  first  obtained  in  1826. 


54        THE  TREASURES  OF  COAL  TAR 

French  dyers  to  whom,  as  we  learn,  the  industrial 
application  of  this  dye  was  largely  due.  "  I 
distinctly  remember/'  said  Sir  William  Perkin 
at  a  later  date,  "  the  first  time  I  induced  a 
calico-printer  to  make  trials  of  this  colour  that 
the  only  report  I  obtained  was  that  it  was  too 
dear,  and  it  was  not  until  nearly  two  years  after- 
wards, when  French  printers  put  aniline  purple 
into  their  patterns,  that  it  began  to  interest 
British  printers/' 

The  successful  industrial  production  of  mauve 
depends  on  the  successful  production  of  nitro- 
benzene from  benzene,  and  of  aniline  from  nitro- 
benzene ;  and  although  these  two  "  intermediates  " 
had  already  been  prepared  in  small  quantities,  their  ' 
production  on  a  large  scale  presented  a  number  of 
difficulties  to  the  pioneers  in  this  industry.  In- 
stead of  the  glass  flasks  in  which,  hitherto,  nitro- 
benzene had  been  prepared  by  the  action  of  fuming 
nitric  acid  on  benzene,  Perkin  employed  a  large 
cast-iron  cylinder,  capable  of  holding  between  thirty 
and  forty  gallons  of  liquid,  and  furnished  with  a 
stirrer  which  could  be  worked  by  a  handle.  Since  a 
sufficiently  large  supply  of  fuming  nitric  acid  could 
not  at  that  time  be  obtained,  Perkin  used  a  mixture 
of  sodium  nitrate  and  concentrated  sulphuric  acid, 
and,  later,  a  mixture  of  concentrated  nitric  and 
sulphuric  acids.  The  conversion  of  nitro-benzene 
into  aniline  was  effected  in  large  iron  stills  by  means 


PRODUCTION  OF  DYES  FROM  COAL  TAR  53 

of  iron  filings  and  acetic  acid,  this  acid,  however, 
being  replaced  at  a  later  date  by  the  cheaper  hydro- 
chloric acid  or  muriatic  acid.  By  the  action  of 
the  acid  on  the  iron,  hydrogen  is  produced,  and  this 
"  reduces  "  the  nitrobenzene,  or  replaces  its  oxygen 
by  hydrogen,  and  so  yields  aniline.  The  methods 
used  at  the  present  day  for  the  manufacture  of  these 
two  very  important  compounds  are  essentially 
those  which  were  introduced  by  Perkin ;  and  in 
thus  working  out  the  details  of  the  process  of  manu- 
facture of  aniline,  now  perhaps  the  most  important 
of  all  the  coal-tar  "  intermediates,"  Perkin  per- 
formed a  service  of  the  highest  value  to  the  coal- 
tar  colour  industry. 

The  success  which  attended  the  introduction  of 
mauve,  the  vogue  of  which  among  the  women  of 
1859  became  so  "  epidemic  "  that  Punch  referred 
to  it  as  "  The  Mauve  Measles,"  naturally  led 
chemists  to  try  the  action  on  aniline  of  other 
oxidising  agents  (substances  capable  of  giving  up 
oxygen  to  other  substances)  than  the  potassium 
dichromate  used  by  Perkin ;  and  although  they 
did  not  succeed  in  displacing  the  latter  for  the  pre- 
paration of  mauve,  their  efforts  led  to  the  discovery 
of  a  new  dye,  aniline  red,  magenta,  or  fuchsine. 
The  formation  of  this  red  dye  had  been  observed  by 
several  chemists,  even  as  early  as  1856,  and  although 
it  was  manufactured  in  Small  quantity  in  France 
in  1858-9,  by  a  process  due  to  Verguin,  the  greatest 


56        THE  TREASURES  OF  COAL  TAR 

success  in  its  manufacture  was  achieved,  in  1860, 
by  two  English  chemists,  Medlock  and  Nicholson, 
former  pupils  of  Hofmann,  who  prepared  it  by  the 
action  of  arsenic  acid  on  commercial  aniline.  The 
manufacture  of  this  important  dye  was  carried  out 
by  Messrs  Simpson,  Maule  and  Nicholson,  and  the 
"  crown "  of  magenta  crystals  prepared  by  this 
firm  was  one  of  the  most  notable  exhibits  of  the 
International  Exhibition  of  1862.  Stirred  by  this 
and  by  the  other  exhibits  of  English  dye  manu- 
facturers, Hofmann  was  prompted  to  make  the 
prediction :  "  England  will,  beyond  question,  at 
no  distant  day  become  herself  the  greatest  colour- 
producing  country  in  the  world,  nay,  by  the  strangest 
of  revolutions,  she  may,  ere  long,  send  her  coal- 
derived  blues  to  indigo-growing  India,  her  tar- 
distilled  crimsons  to  cochineal-producing  Mexico, 
and  her  fossil  substitutes  for  quercitron  and  safflower 
to  China,  Japan,  and  the  other  countries  whence 
these  articles  are  now  derived."  At  that  time 
England  was  pre-eminent  in  the  industrial  pro- 
duction of  the  coal-tar  dyes  as  she  was  pre-eminent 
in  the  production  of  the  raw  materials  of  their 
manufacture,  but  in  the  subsequent  years,  largely 
through  her  failure  to  recognise  the  vital  importance 
of  persistent  chemical  research,  she  had  to  yield 
pride  of  place  to  Germany.  Let  us,  however,  still 
hope  that  the  faith  in  science  which  has  been 
awakened  in  the  people  of  this  country  during  the 


PRODUCTION  OF  DYES  FROM  COAL  TAR  57 

years  of  war  will  yet  enable  us  in  the  years  of 
peace  to  regain  our  lost  position  and  so  realise  the 
prophecy  made  by  Hofmann  in  1862. 

The  brilliancy  of  the  new  aniline  dyes  and  the 
great  success  they  achieved  owing,  partly,  to  the 
simplification  which  their  use  brought  about  in 
the  process  of  dyeing,  made  a  very  powerful  appeal 
to  the  imagination  of  the  scientific  chemist  no  less 
than  to  the  business  instincts  of  the  manufacturer. 
"  A  new  world  was  disclosed  full  of  magic  promise, 
and  all  joined  eagerly  in  the  search,  the  manufac- 
turer and  the  professor,  the  business  man  and  the 
adventurer ;  for  the  one  a  new  gold-mine,  for  the 
other  new  opportunities  of  fruitful  investigation/' 
To  such  an  extent,  indeed,  were  the  energies  of 
chemists  directed  along  this  one  channel  that  it 
was  feared  by  some  that  the  general  progress 
of  chemical  science  would  be  gravely  prejudiced. 
But  the  check,  if  any,  was  but  temporary,  for 
the  co-operation  which  then  existed  between  the 
scientific  investigator  and  the  chemical  technolo- 
gist, a  co-operation  which  this  country  must 
endeavour  once  more  to  re-establish,  proved  most 
beneficial ;  and  the  new  materials  which  the 
manufacturer  soon  placed  at  the  disposal  of  the 
investigator  led  to  a  far  greater  extension  of 
chemical  science  than  would  otherwise  have  been 
possible. 

Just  as  aniline  formed  the  basis  of  manufacture 


58        THE  TREASURES  OF  COAL  TAR 

of  rnauve  and  of  magenta,  so  magenta  became,  in 
its  turn,  the  starting-point  for  the  preparation  of 
a  series  of  new  dyes,  the  number  of  which  now  began 
rapidly  to  increase.  In  1861  Girard  and  de  Laire 
prepared  aniline  blue  or  Lyons  blue  by  heating 
magenta  with  aniline  in  presence  of  benzoic  acid ; 
and  by  treating  this  dye  with  concentrated  sulphuric 
acid,  E.  C.  Nicholson,  in  1862,  produced  the  more 
valuable  Nicholson's  blue  or  water  blue,  which 
possessed  the  great  advantage  of  being  soluble 
in  water  and  in  solutions  of  alkalies,  and  was 
better  adapted  for  dyeing  wool  than  the  dyes  pre- 
viously prepared.  Hofmann,  also,  prepared  brilliant 
but  not  very  stable  violet  dyes,  Hofmann  violets, 
by  acting  on  magenta  with  methyl  iodide  and  ethyl 
iodide. 

But  although  the  preparation  of  new  dyes  and 
the  perfecting  of  their  industrial  production  were 
carried  on  with  much  vigour  along  the  lines 
opened  up  by  W.  H.  Perkin,  chemists  were  not 
unmindful  of  the  need  of  more  theoretical  in- 
vestigations for  the  purpose  of  determining  the 
composition  and  unravelling  the  constitution  of 
these  important  new  substances.  Without  such 
knowledge  the  dye  industry  could  not  be  placed 
on  a  secure  scientific  basis  and  its  further 
development  ensured.  In  this  work  of  investiga- 
tion Hofmann  took  a  leading  part,  and  in  1862 
he  showed  that  the  dye  magenta  was  the  salt  of 


PRODUCTION  OF  DYES  FROM  COAL  TAR  59 

a  base  l  which  he  called  rosaniline.  Moreover,  in 
1864  he  confirmed  what  had  already  been  dis- 
covered by  Nicholson,  that  magenta  cannot  be 
obtained  by  the  oxidation  of  pure  aniline  but  only 
of  commercial  aniline  which  contained  the  two 
isomeric  substances,  ortho-  and  para-toluidine,  as 
impurities. 

We  have  already  seen  that  coal-tar  contains  not 
only  benzene  but  also  several  other  similar  hydro- 
carbons, more  especially  toluene  ;  and  in  the  early 
days  of  the  industry  the  separation  of  these  was  not 
carried  out  very  effectively.  In  other  words,  the 
benzol  obtained  by  the  distillation  of  the  coal-tar 
always  contained  larger  or  smaller  amounts  of 
toluene.  When  this  commercial  benzol  was  treated 
with  a  mixture  of  nitric  and  sulphuric  acids  there 
were  produced  not  only  nitrobenzene  but  also  two 
isomeric  nitro-toluenes,  namely,  ortho-  and  para- 
nit  rot  oluene  (see  p.  44)  : 

CH3  CH3 


N02 
Ortho-nitrotoluene  Para-nitrotoluene 

and  when  these  nitrotoluenes  are  "  reduced  "  by 

1  Chemists  are  accustomed  to  classify  substances  into  acids, 
bases,  and  salts.  An  acid  is  a  substance  with  a  sharp,  sour,  or 
acid  taste  (e.g.  vinegar  or  acetic  acid),  which  can  combine  with 
or  neutralise  a  base  (e.g.  ammonia  or  soda),  with  production  of 
a  "  neutral  "  substance,  a  salt. 


60        THE  TREASURES  OF  COAL  TAR 

means  of  iron  and  hydrochloric  acid,  the  two  cor- 
responding toluidines  are  produced : 

CH3  CH3 


NH2 
Ortho-toluidine  Para-toluidine 

It  was  to  the  presence  of  these  compounds  in  the 
aniline  employed  that  the  discovery  of  magenta, 
as,  indeed,  also  of  mauve,  was  due. 

But  although  Hofmann  succeeded  in  determining 
the  composition  of  magenta  and  of  some  of  the 
other  dyes  then  known,  the  true  relationships 
which  existed  between  these  dyes  could  not  be 
understood  without  a  knowledge  of  the  structure 
or  constitution  of  the  molecule.  This  knowledge, 
made  possible  by  the  theories  of  structure  put 
forward  by  Kekule  (p.  36),  was  finally  obtained 
in  1878  by  the  two  German  chemists  Emil  and 
Otto  Fischer,  who  showed  that  the  parent  of  rosani- 
line,  magenta,  and  a  number  of  other  dyes  derived 
from  aniline,  is  a  hydrocarbon  called  triphenyl- 
methane.  This  compound  can  be  regarded  as 

H 
arising  from  methane,  H-C-H,  by  the  replacement 

H 

of  three  of  the  hydrogen  atoms  by  the  group 
phenyl  or  C6H5,  that  is,  a  molecule  of  benzene  from 
which  one  hydrogen  atom  has  been  removed.  We 


PRODUCTION  OF  DYES  FROM  COAL  TAR  61 

therefore  obtain   as   the   formula   of   this    parent 
hydrocarbon : 


/ 

H-C <  y     or     H-C—  C6H5 


When  a  mixture  of  aniline,   ortho-toluidine,   and 
para-toluidine  is  oxidised,  rosaniline  is  produced  : 


CH 

NH2 


NH2     or      HO-C—  C6H4-NH2 
NH2 


(the  relation  of  which  to  the  parent  hydrocarbon, 
triphenyl-methane,  is  readily  seen),  and  this  base 
unites  with  hydrochloric  acid,  with  elimination  of 
water,  to  yield  rosaniline  hydrochloride  or  magenta  : 

,C6H3(CH3)-NH2 
C—  C6H4'NH2 


When  a  mixture  of  aniline  and  para-toluidine 
is  oxidised,  another  base,  para-rosaniline,  is 
obtained,  and  this  also  gives  rise  to  dyes  similar 


62        THE  TREASURES  OF  COAL  TAR 

to  those  given  by  rosaniline,  to  which  they  are, 
structurally,  closely  related,  as  the  formulae  show  : 

//C6H4-NH2  //C6H4-NH2 

HO'C— C6H4'NH2  C— C6H4'NH2 

X6H4-NH2  ^C6H4-NH2C1 

Para-rosaniline  Para-rosaniline  hydrochloride 

(red  dye) 

The  theory  of  the  structure  of  the  benzene  mole- 
cule put  forward  by  Kekule,  and  the  elucidation 
of  the  constitution  of  rosaniline  and  para-rosaniline 
which  it  rendered  possible,  not  only  enabled  one  to 
understand  the  exact  relations  between  the  different 
dyes,  the  Hofmann  violets,  aniline  blue,  etc.,  which 
had  already  been  prepared,  but  new  and  better 
processes  for  the  synthesis  of  these  and  other 
dyes  could  be  introduced.  Thus  the  old  process  for 
the  manufacture  of  para-rosaniline  and  rosaniline 
(magenta)  has  given  place  to  the  "  New  Fuchsine  " 
process  in  which  para-rosaniline  is  prepared  from 
formaldehyde  and  aniline,  while  rosaniline  is  pre- 
pared from  formaldehyde,  aniline,  and  ortho- 
toluidine. 

The  hydrogen  atoms  of  the  three  NH2-groups 
present  in  rosaniline  and  para-rosaniline  can  be 
replaced  by  various  groups,  such  as  methyl  (CH3), 
ethyl  (C2H5),  phenyl  (C6H5),  benzyl  (C6H5-CH2), 
etc.,  by  acting  on  the  compounds  with  methyl 
chloride,  ethyl  chloride,  aniline,  benzyl  chloride, 
etc.  In  this  way  whole  series  of  dyes  can  be 


PRODUCTION  OF  DYES  FROM  COAL  TAR  63 

obtained,  like  the  Hofmann  violets,  which  were 
prepared  by  replacing  three  hydrogen  atoms  in 
rosaniline  by  methyl  or  ethyl  groups ;  and  aniline 
blue,  which  is  derived  from  magenta  by  the  replace- 
ment of  three  hydrogen  atoms  by  phenyl  groups. 
Since,  therefore,  a  varying  number  of  hydrogen 
atoms  can  be  replaced  by  different  groups,  it  will 
readily  be  understood  that  from  the  parent  dye, 
magenta,  quite  a  considerable  number  of  derived 
dyes  can  be  prepared. 

The  production  of  the  Hofmann  violets,  we  have 
seen,  involves  the  introduction  of  new  reagents, 
methyl  and  ethyl  iodide  or  chloride,  and  for  the 
preparation  of  these,  methyl  alcohol  or  "  wood 
spirit  "  and  ethyl  alcohol  or  "  spirits  of  wine  "  are 
required.  The  development  of  the  dye  industry, 
therefore,  depended  on  and  was  accompanied  by 
the  development  of  other  chemical  industries ; 
and  the  industrial  production  of  the  large  number 
of  different  compounds  used  in  the  manufacture  of 
dyes  becomes  a  matter  of  supreme  importance  for 
the  success  of  dye-manufacture.  By  the  introduc- 
tion of  new  reagents,  moreover,  other  new  com- 
pounds, other  intermediates,  can  be  prepared,  and 
these,  in  turn,  may  become  the  starting-point  for 
other  series  of  dyes,  the  dye  industry  thereby  grow- 
ing rapidly  in  diversity  as  a  tree  grows  by  the  rami- 
fication of  its  branches.  Thus,  by  heating  aniline 
with  methyl  chloride,  the  compound  dimethyl- 


64        THE  TREASURES  OF  COAL  TAR 

aniline  is  produced,  and  by  acting  on  this  with  an 
oxidising  agent  a  beautiful  violet  dye,  known  as 
methyl  violet,  is  obtained,  and  is  largely  used  as  a 
staining  liquid  in  microscopy  and  in  the  manu- 
facture of  indelible  pencils.  It  consists  of  a  mixture 
of  dyes  which  may  be  regarded  as  derived  from 
para-rosaniline  by  the  replacement  of  four,  five, 
and  six  atoms  of  hydrogen  by  methyl  groups.  The 
pure  hexamethyl  derivative  of  para-rosaniline  (with 
six  atoms  of  hydrogen  replaced  by  methyl  groups) 
is  obtained  from  dimethyl-aniline  and  phosgene 
(from  carbon  monoxide  and  chlorine),  and  is  known 
as  crystal  violet.  By  the  action  of  methyl  chloride 
on  methyl  violet,  a  brilliant  green  dye,  methyl 
green,  is  obtained. 

The  production  of  new  dyes  by  the  replace- 
ment of  hydrogen  by  different  groups  of  atoms 
leads  us  to  the  consideration  of  a  subject  of  the 
highest  importance,  the  relation  between  colour 
and  constitution. 

Put  in  general  terms,  a  substance  possesses  colour 
only  when  it  has  the  power  of  absorbing  light  of  a 
certain  wave-length  while  allowing  light  of  other 
wave-length  to  pass  through.  When  a  substance 
absorbs  the  green  rays,  for  example,  the  light  which 
passes  through  will  show  the  complementary  colour, 
namely,  purple  red.  In  other  words,  the  substance 
will  appear  of  this  colour. 


PRODUCTION  OF  DYES  FROM  COAL  TAR  65 

From  the  study  of  a  large  number  of  substances 
the  conclusion  has  been  reached  that  colour  is 
associated  with  the  presence  of  certain  groups 
or  arrangements  of  atoms  in  the  molecule — such 
groups  being  known  as  "  chromophors  "  or  colour- 
bearing  groups.1  In  the  dyes  which  we  have 
already  mentioned  the  colour  is  believed  to  be  due 
to  a  modification  of  one  of  the  benzene  rings.  But 
although  the  character  of  a  dye-stuff  is  derived 
from  its  chromophor,  the  actual  colour  and  shade 
may  be  very  distinctly  altered  by  the  introduction 
of  different  groups  into  the  molecule.  Thus,  if  we 
write  down  the  different  primary  colours,  namely, 

Greenish-yellow, 

Yellow, 

Orange, 

Red, 

Purple, 

Violet, 

Indigo, 

Cyanide-blue, 

Bluish-green, 

it  is  found  that  the  introduction  of  methyl  and 
ethyl  groups,  and  still  more  the  introduction  of 

1  When  a  chromophor  is  present  in  a  compound  the  latter  is 
said  to  be  a  "  chromogen,"  and  may  or  may  not  be  coloured. 
To  convert  the  latter  into  a  coloured  substance,  it  is  necessary 
to  introduce  certain  groups  (especially  OH  and  NH2),  called 
"  auxochromes." 


66        THE  TREASURES  OF  COAL  TAR 

phenyl,  benzyl,  and  other  groups  derived  from 
benzene,  produces  a  change  of  colour  in  the  direc- 
tion shown  by  the  arrow.  This  fact  is  well  illus- 
trated by  the  dyes  which  have  already  been  men- 
tioned, for  by  introducing  three  methyl  or  ethyl 
groups  into  the  molecule  of  magenta  (red),  Hofmann 
obtained  violet  dyes.  Moreover,  by  increasing  the 
number  of  such  groups  the  violet  shade  becomes 
bluer,  as  is  shown  in  the  case  of  methyl  violet  and 
crystal  violet,  which  contain  five  or  six  methyl 
groups.  But,  as  we  have  said,  the  phenyl  group 
has  a  more  powerful  effect  than  the  methyl  or  ethyl 
group,  and  so  we  find  that  when  it  is  introduced 
into  a  molecule  in  place  of  hydrogen,  the  alteration 
of  shade  or  colour  is  much  greater.  This  is  illus- 
trated by  aniline  blue,  which  is  obtained  by  replacing 
three  hydrogen  atoms  in  magenta  by  three  phenyl 
groups.  Since,  by  the  introduction  of  certain  other 
groups,  the  shade  or  colour  can  be  altered  in  the 
opposite  direction  to  that  indicated  by  the  arrow, 
it  will  readily  be  understood  how  it  becomes  possible 
not  only  to  prepare  a  considerable  number  of  parent 
dyes,  but  also,  from  these  dyes,  to  produce,  at  will, 
a  great  variety  of  shades  and  colours  simply  by  the 
introduction  of  different  groups  into  the  molecule 
of  the  parent  dye. 

To  attempt  a  full  discussion  of  all  the  dye  deriva- 
tives of  triphenyl-methane  would  be  impossible 
within  the  limits  of  space  available  here,  and  would, 


PRODUCTION  OF  DYES  FROM  COAL  TAR  67 

moreover,  be  a  very  tedious  matter  for  the  general 
reader.  But  reference  may  be  made  to  one  other 
dye  of  importance,  discovered  in  1878.  It  has 
already  been  noted  that  as  a  result  of  the  intro- 
duction into  the  dye  industry  of  methyl  chloride 
and  iodide,  the  preparation  of  dimethyl-aniline 
[C6H5-N(CH3)2]  became  possible.  Similarly,  in  place 
of  the  two  methyl  (CH3)  groups,  other  groups  can 
be  introduced  into  aniline,  as  has  already  been 
indicated,  and  from  these  derivatives  of  aniline 
other  series  of  dyes  can  be  obtained  by  the  action 
of  benzaldehyde  or  bitter  almond  oil.  Thus, 
dimethyl-aniline  gives  rise  to  the  well-known  dye, 
malachite  green  or  Victoria  green,  and  diethyl- 
aniline  [C6H5-N(C2H5)2]  to  the  dye  brilliant  green. 

The  synthetic  production  of  these  two  and  other 
dyes  necessitating  the  use  of  benzaldehyde  depended 
for  industrial  success  on  the  discovery  of  a  cheaper 
and  better  source  of  this  compound  than  that 
already  existing.  Hitherto,  benzaldehyde  had  been 
obtained  by  the  fermentation  of  bitter  almonds. 
In  this  process  the  compound,  amygdalin,  present 
in  the  almonds,  undergoes  a  decomposition  with 
production  of  benzaldehyde  and  certain  other  sub- 
stances. At  the  present  day,  however,  this  com- 
pound, formerly  obtainable  only  from  vegetable 
sources,  is  manufactured  in  large  quantities  from 
the  coal-tar  hydrocarbon,  toluene ;  and  from  this 


68        THE  TREASURES  OF  COAL  TAR 

same  source  is  also  obtained  the  compound  benzoic 
acid,  which,  as  we  have  seen,  is  used  in  the  manu- 
facture of  aniline  blue  (p.  58).  Thus  the  dye 
industry  goes  on  increasing  in  diversity,  drawing 
into  its  service  one  compound  after  another,  and 
depending,  therefore,  not  only  on  the  persistent  and 
untiring  work  of  the  research  chemist  but  also  on 
the  success  with  which  the  technologist  can  carry 
out  on  an  industrial  scale  the  discoveries  of  the 
investigator.  And  any  country  which  aims  at 
developing  a  dye-making  industry  which  will  render 
it  independent  of  other  countries  must  learn  to 
produce,  with  the  utmost  economy,  the  large 
number  of  "  intermediates  "  of  which  that  industry 
makes  use. 

The  dyes  which  have  so  far  been  mentioned  will 
dye  silk  and  wool  directly,  but  will  dye  cotton  only 
with  the  aid  of  a  mordant.  Although  not  character- 
ised by  great  fastness  to  light,  the  dyes  of  the  tri- 
phenyl-methane  series  possess,  for  the  most  part, 
great  brilliance,  and  are,  in  consequence,  much 
esteemed  for  the  dyeing  of  those  materials — ribbons, 
for  example — for  which  brilliancy  is  more  valued 
than  fastness.  Owing  to  the  introduction  of  other 
series  of  dyes,  however,  the  further  development 
of  the  triphenyl-methane  dyes  has  practically 
ceased. 

The  relations  between  the  different  dyes  men- 


PRODUCTION  OF  DYES  FROM  COAL  TAR  69 

tioned  in  this  chapter  are  shown  in  the  following 
diagram  l : 


Crystal                Methyl 
Violet                 Violet 
I                           1 

Malachite 
Green 
1    1 

lehyde 

Dime 
ani 

thyl- 
ine 
Hofmann 
Violets 

Aniline 
Blue 
1    1 

Magenta                        Benzoic        Benzal< 
|    |                               Acid 

Ani 

Nitrob 

Ben 

line 
enzene 

sene 

Toluidines 
1 
Nitrotoluenes 
I 

I 
Toluene 

1  For  a  more  complete  representation  of  the  relationships 
between  the  coal-tar  crudes,  intermediates,  and  finished  dyes, 
the  reader  is  referred  to  the  Chart  drawn  up  by  Thomas  H. 
Norton  and  published  by  Messrs  George  Allen  &  Unwin,  Ltd., 
40  Museum  Street,  London,  W.C. 


CHAPTER  VI 

AZO-DYES 

SOME  years  after  the  epoch-making  discovery  of 
mauve  an  observation  was  made  which  led  in  time 
to  the  development  of  an  entirely  new  class  of  com- 
pounds which,  in  number  and  importance,  now 
occupy  a  foremost  place  among  coal-tar  dyes.  In 
1860  it  was  found  by  Dr  Peter  Griess,  chemist  in 
the  brewery  of  Messrs  Allsopp  &  Sons,  Burton-on- 
Trent,  that  when  nitrous  acid  acts  on  aniline,  or 
on  any  other  derivative  of  benzene  containing  the 
amino-group  (NH2),  an  unstable  compound — a  so- 
called  diazo-compound — is  produced.  The  diazo- 
compounds  which  were  thus  obtained  possessed 
the  very  important  property  of  combining  or 
"  coupling  "  with  aromatic  amines  (compounds  con- 
taining the  NH2-group),  or  with  phenols  (compounds 
containing  the  OH-group).  In  this  way  were  pro- 
duced the  so-called  azo-dyes,  which  have  found  their 
special  application  as  wool-dyes,  and  which  owe 
their  colour  to  the  presence  of  the  chromophoric 
azo-group  or  pair  of  linked  nitrogen  atoms,  — N :  N — . 
Even  in  1863,  although  its  constitution  was  not  then 
known,  an  azo-dye,  aniline  yellow,  had  been  prepared 

70 


AZO-DYES  71 

and  put  on  the  market,  with  only  a  limited  success  ; 
but  it  was  not  till  1876,  and  after  the  constitution  of 
the  compounds  had  been  elucidated,  that  the  pro- 
duction of  azo-dyes  began  to  undergo  a  rapid  de- 
velopment. Many  hundreds,  even  thousands,  of 
azo-dyes  have  now  been  prepared,  and  a  very  con- 
siderable number  have  been  found  suitable  for  use 
as  dyes. 

When  aniline  is  "  diazotised  "  by  the  action  of 
nitrous  acid,  and  the  resulting  compound  then 
"  coupled  "  with  a  molecule  of  aniline,  a  compound 
is  obtained  which  can  be  represented  by  the  graphic 
formula 

&••• 

This  is  a  basic  substance  and  can  form  a  salt  with 
hydrochloric  acid,  yielding  thereby  the  dye  aniline 
yellow,  a  dye  which,  on  account  of  its  fugitive 
nature,  is  now  no  longer  used  except  in  the  manu- 
facture of  other  dyes.  If,  however,  aniline  yellow 
is  treated  with  a  highly  concentrated  sulphuric 
acid,  two  sulphonic  acid  groups  (SO2OH)  enter  the 
molecule,  and  a  more  stable  dye,  acid  yellow,  is 
obtained.  This  process  of  sulphonation,  as  was 
pointed  out  by  the  late  Sir  W.  H.  Perkin,  is  one 
of  the  highest  importance  in  the  production  of 
stable  dyes. 

If,  after  diazotising  aniline  one  couples  the  pro- 
duct, not  with  aniline  but  with  a  benzene  deriva- 


72        THE  TREASURES  OF  COAL  TAR 

live  containing  two  ammo-groups,  namely,  meta- 
phenylene  diamine, 

NH2 


JNH2 

one  obtains  the  compound 

N— <f  >NH2 


the  salt  of  which  with  hydrochloric  acid  constitutes 
the  orange-red  dye  chrysoidine.  Or,  again,  if  one 
diazotises  meta-phenylene  diamine  and  couples 
the  product  with  another  molecule  of  this  compound, 
there  is  formed 


>N  :  N<T  >NH2 

NHa  NHa 

the  salt  of  which  with  hydrochloric  acid  constitutes 
Bismarck  brown. 

But  we  can  couple  the  diazo-compounds  not  only 
with  compounds  containing  the  amino-group,  but 
also  with  compounds  containing  the  hydroxyl- 
group  (OH),  e.g.  phenol,  cresol,  salicylic  acid,  etc., 
and  in  this  way  another  series  of  azo-dyes  is  obtained 
known  as  the  tropaeolines.  Thus,  for  example, 
if  one  diazotises  not  aniline  itself  but  the  sulphonic 
acid  derivative  of  aniline,  known  as  sulphanilic 

acid,  SO2OH<^ />NHa,  and  if  one  then  couples  the 


AZO-DYES  73 

product  with  the  compound  resorcinol,  <(^      _/OH 

OH 

(a  substance  derived  from  benzene),  there  is  obtained 
the  compound 


N  :  N  OH 

OH 

a  dye  known  as  tropseoline  0. 

Not  only  can  one  produce  azo-dyes  from  aniline, 
toluidine,  phenol,  etc.,  and  from  their  derivatives, 
but  one  may  also  use  similar  compounds  derived 
from  other  hydrocarbons.  In  this  connection  the 
coal-tar  hydrocarbon  naphthalene  has  been  found  of 
especial  value,  and  very  many  dyes  have  now  been 
produced  from  the  amino-  and  hydroxyl-derivatives 
of  this  compound.  By  thus  drawing  naphthalene 
within  the  sphere  of  the  dye  industry,  an  important 
outlet  was  secured  for  this  coal-tar  product  —  other- 
wise but  little  used  —  and  at  the  same  time  a  large 
number  of  valuable  new  dyes  were  obtained. 

Just  as  we  have  seen  that  phenol  and  aniline  are 
derived  from  benzene  by  the  replacement  of  a 
hydrogen  atom  by  a  hydroxyl-  or  amino-group,  so 
from  naphthalene  one  can  obtain  similar  com- 
pounds —  naphthol  and  naphthylamine.  Owing 
to  the  peculiar  constitution  of  naphthalene,  how- 
ever, two  isomeric  naphthols  and  naphthylamines 
can  be  obtained,  known  as  alpha-naphthol  and 
beta-naphthol,  alpha-naphthylamine  and  beta- 


74        THE  TREASURES  OF  COAL  TAR 

naphthylamine.  These  very  important  com- 
pounds, as  well  as  their  sulphonic  acid  derivatives, 
are  now  manufactured  in  large  quantities  and  used 
in  the  production,  more  especially,  of  azo-dyes. 
By  diazotising  aniline,  toluidine,  xylidine,  etc., 
and  coupling  the  products  with  the  sulphonic  acid 
derivative  of  beta-naphthol,  one  obtains  a  series 
of  red  dyes  known  as  Ponceaux,  the  shade  varying 
according  to  the  amino-compound  (aniline,  toluidine, 
etc.)  employed.  And,  similarly,  other  dyes  can 
be  obtained  by  diazotising  the  naphthylamines  or 
their  derivatives,  and  coupling  the  products  with 
various  amino-compounds,  phenols,  naphthols,  etc. 
Moreover,  in  those  cases  where  a  diazo-compound 
is  coupled  with  an  amino-compound,  the  process 
of  diazotisation  can  be  repeated,  and  dyes  contain- 
ing two  azo-groups,  e.g.  Biebrich  scarlet,  can  thus 
be  obtained.  Many  of  these  are  of  great  importance. 
From  what  has  now  been  said,  some  idea  will  be 
gained  not  only  of  the  large  number  of  azo-dyes 
which  can  be  obtained,  but  also  of  the  increasing 
number  of  "  intermediates  "  made  use  of  by  the 
dye  industry.  Moreover,  for  the  production  of  all 
these  azo-dyes,  nitrous  acid  is  necessary  for  carry- 
ing out  the  first  step  in  the  process,  that  is  to 
say,  the  diazotising  of  the  initial  amino-compound. 
This  nitrous  acid  is  produced  from  sodium  nitrite, 
and  this,  in  turn,  is  obtained  by  heating  sodium 
nitrate  with  lead.  Until  recently,  one  was  depend- 


AZO-DYES  75 

ent  for  sodium  nitrate  on  the  large  deposits  of  this 
salt  in  Chile,  but  in  the  past  decade  the  production 
of  nitric  acid  directly  from  the  air  has  opened  up 
a  new  source  of  supply  of  this  important  salt.  In 
this  matter  of  the  utilisation  of  atmospheric  nitrogen 
for  the  production  of  nitric  acid  and  other  com- 
pounds of  nitrogen,  this  country  has  lagged  behind 
not  only  Germany  but  nearly  every  other  civilised 
country  in  the  world.  There  seems,  however,  to  be 
now  some  hope  that  this  past  neglect  will  be  repaired. 

Although  most  of  the  dyes  to  which  reference  has 
already  been  made,  dye  silk  and  wool  directly, 
vegetable  fibres,  e.g.  linen  and  cotton,  must  first 
be  mordanted  before  they  will  take  up  the  dye 
from  the  bath.  It  was  therefore  an  event  of  the 
first  importance  when,  in  1884,  the  German  chemist 
Bottinger  discovered  a  new  group  of  azo-dyes 
which  were  able  to  dye  cotton  and  linen  directly 
without  requiring  a  mordant.  In  this  discovery 
is  to  be  found  undoubtedly  one  of  the  reasons  for 
the  outstanding  importance  of  the  azo-dyes. 

These  direct  cotton  dyes  contain  two  azo-groups 
or  two  pairs  of  nitrogen  atoms,  and  are  derived 
from  compounds  similar  to  benzidine  and  tolidine, 
in  which  there  are  two  benzene  rings  joined  together, 
thus: 


NH'<Z>-<Z>NHa     NH.OO^H, 


Benzidine  Tolidine 


76        THE  TREASURES  OF  COAL  TAR 

When  these  compounds,  which  are  prepared  from 
benzene  and  toluene  respectively,  are  acted  on  by 
nitrous  acid,  both  NH2-groups  are  diazotised,  and 
the  product  can  then  couple  with  two  molecules 
of  amine  or  phenol.  When  benzidine,  for  example, 
is  diazotised  and  the  product  coupled  with  two 
molecules  of  sulphonated  naphthylamine,  the  red 
dye,  Congo  red,  is  obtained.  This  was  the  first 
direct  cotton  dye  to  be  synthesised,  and  led  to 
the  preparation  of  a  large  number  of  similarly 
constituted  dyes,  covering  the  whole  range  of 
colour,  all  of  which  possess  the  valuable  property 
of  dyeing  cotton  without  the  aid  of  a  mordant. 
The  chart  on  the  following  page  indicates  the 
relationship  of  some  of  the  azo-dyes  to  the 
hydrocarbon  benzene. 

The  simplest  azo-dyes  are  yellow,  but,  as  has 
already  been  pointed  out,  the  shade  and  colour 
can  be  altered  very  greatly  by  the  introduction  of 
different  groups.  In  this  way  dyes  of  deeper  and 
deeper  tone,  from  the  vivid  scarlets  known  as 
xylidine  scarlet  and  Biebrich  scarlet — successful 
rivals  of  the  natural  dye  cochineal — to  blue,  violet, 
brown,  and  black,  have  been  produced ;  and  this 
change  of  colour  is  effected,  more  especially,  by  the 
introduction  of  groups  derived  from  naphthalene. 
By  the  introduction  of  nitro-groups  (NO2),  green 
dyes,  e.g.  diamine  green,  are  obtained. 

As  a  result  of  the  introduction  of  the  azo-dyes 


AZO-DYES 


77 


78        THE  TREASURES  OF  COAL  TAR 

an  important  development  in  the  art  of  dyeing  has 
taken  place.  If  the  fabric  to  be  dyed  is  first  im- 
pregnated with  an  alkaline  solution  of  beta-naphthol 
and  then  immersed  in  a  solution  of  diazotised 

para-nitraniline  (NO2<^       yNH2),  the   dye,  para- 

nitr aniline  red  or  para- red,  is  produced  on  the  fibre. 
Dyeing  with  this  pigment  dye — as  a  dye  produced 
on  the  fibre  is  called — is  carried  out  on  a  very  large 
scale,  nearly  two  thousand  tons  of  para-nitraniline 
being  produced  annually  for  the  purpose.  This 
process  of  dyeing  can,  similarly,  be  carried  out  with 
dyes  containing  an  amino-group  which  can  be 
diazotised  on  the  fibre.  Thus  there  is  a  very  com- 
plex dye  primuline,  discovered  by  Professor  A.  G. 
Green  in  1887,  which  will  dye  cotton  directly  of  a 
yellow  shade.  This  dye  is  somewhat  fugitive,  and 
is,  consequently,  of  comparatively  little  value  in 
itself.  If,  however,  a  piece  of  calico,  dyed  with 
primuline,  is  passed  through  a  cold,  dilute  solution 
of  nitrous  acid,  the  primuline  (which  contains  an 
amino-group),  is  diazotised ;  and  if  the  fabric  is 
then  passed  through  a  solution  of  an  amine  or 
phenol,  a  dye  is  produced  or  "  developed  "  on  the 
fibre.  Thus,  with  beta-naphthol,  primuline  red; 
with  resorcinol,  primuline  orange ;  with  meta- 
phenylene-diamine,  primuline  brown ;  and  with 
salicylic  acid,  oriol  yellow  is  obtained.  To  these 
colours  developed  on  the  fibre  the  name  of  "  in- 


AZO-DYES  79 

grain  dyes  "  is  given ;  or,  since  the  solution  of  the 
diazo-compound  has,  on  account  of  its  instability, 
to  be  kept  cool  by  means  of  ice,  the  name  "  ice 
colours  "  is  also  sometimes  applied  to  this  group 
of  dyes. 

Another  pigment  dye  to  which  we  may  here  refer, 
a  dye  which  carries  us  back  again  to  the  substance, 
aniline,  from  which  the  first  artificial  colouring 
matter  was  prepared,  is  the  very  important  black 
dye,  aniline  black.  This  dye,  which  has  a  very 
complex  structure,  and  is  not  an  azo-dye,  is 
obtained  by  a  modification  of  the  process  first 
used  by  Perkin  in  the  production  of  mauve,  the 
dye  being  formed,  however,  directly  on  the  fibre. 
Thus,  if  a  piece  of  cotton  is  first  steeped  in 
a  solution  of  aniline  in  hydrochloric  acid  and 
afterwards  immersed  in  a  cold  solution  of  sodium 
bichromate,  a  fast  black  colour  is  developed 
on  the  fibre.  Various  improvements  have  more 
recently  been  introduced  for  the  purpose  both  of 
cheapening  the  dye  and  of  obtaining  more  readily 
a  black  which  will  not  turn  green  ;  and  it  has  been 
found  by  Professor  A.  G.  Green  that  in  the  presence 
of  certain  substances  the  oxidation  of  the  aniline 
may  even  be  effected  by  atmospheric  oxygen. 


CHAPTER  VII 

ANTHRACENE  DYES  AND  VAT  DYES 

Ws  have  already  seen  how,  owing  to  the  introduc- 
tion of  the  azo-dyes,  the  coal-tar  hydrocarbon 
naphthalene,  through  its  derivatives  the  naph- 
thylamines,  the  naphthols,  and  their  sulphonic 
acids,  became  one  of  the  important  raw  materials 
in  the  coal-tar  dye  industry.  In  a  like  manner,  as 
a  result  of  the  all-transforming  genius  of  the  chemist, 
another  coal-tar  hydrocarbon,  anthracene,  has  also 
been  made  to  play  a  role  of  the  highest  importance 
in  the  production  of  colouring  matters.  This  hydro- 
carbon, as  we  have  already  seen  (p.  43),  has  the 
formula  C14H10,  and  can  be  represented  graphically 
by  three  rings  joined  together,  thus  : 


By  oxidation  this  compound  is  readily  converted 
into  an  orange-coloured  substance,  anthraquinone, 

CH     CO     CH  r> 


I       -II        II        I 
HC      C       C       CH 


or 


ANTHRACENE  DYES  AND  VAT  DYES    81 

This  structure  forms  the  nucleus  of  a  considerable 
number  of  important  dyes,  and  more  especially 
of  alizarin,  the  colouring  matter  of  the  madder. 
This  dye,  which  is  capable  of  dyeing  cotton  of  a 
bright  red  colour — the  so-called  Turkey  red — is  one 
of  the  oldest  and  best -known  dye-stuffs  employed 
by  man  (to  which,  indeed,  its  use  in  the  dyeing 
of  Egyptian  mummy-cloths  bears  witness)  ;  and, 
although  now  partly  displaced  by  the  azo-dyes,  it 
is  still  very  extensively  used  for  the  dyeing  of  cotton 
goods.  Fifty  years  or  so  ago,  in  the  South  of  France 
and  extending  eastwards  to  Asia  Minor,  great 
tracts  of  land,  about  400,000  acres  in  extent,  were 
devoted  to  the  cultivation  of  the  madder  plant 
(Rubia  tinctoria],  and  produced  about  80,000  tons 
annually  of  madder  roots.  When  these  roots  are 
crushed  and  allowed  to  ferment  certain  compounds 
which  they  contain,  known  as  glucosides,  undergo 
decomposition  with  production  of  the  sugar  glucose 
and  various  colouring  matters,  of  which  the  most 
important  are  alizarin — so  called  from  the  name, 
alizari,  given  by  the  Arabs  to  the  madder  root — 
and  purpurin,  dyes  which  were  first  isolated  in 
1826  by  the  French  chemists  Robiquet  and  Colin. 
But  in  1868  far-reaching  economic  changes  were 
initiated  by  the  discovery,  due  to  Graebe  and 
Liebermann,  of  the  chemical  nature  of  alizarin  and 
by  its  artificial  production  from  what  was  then  a 
waste  by-product  of  the  distillation  of  coal,  the 
F 


82        THE  TREASURES  OF  COAL  TAR 

hydrocarbon  anthracene ;  and  in  1869  the  com- 
mercial production  of  alizarin  from  anthracene  was 
commenced  by  Perkin  in  England.  Since  that 
time  the  natural  dye-stuff  has  been  completely 
superseded  by  the  synthetic,  and  the  widespread- 
ing  lands  over  which  the  madder  once  bloomed 
are  covered  now  with  other  crops  ;  and  an  industry 
which  was  valued  at  about  £4,000,000  annually 
has  passed  from  the  field  to  the  factory.  By  this 
revolution,  also,  anthracene,  which  once  tar  dis- 
tillers did  not  trouble  to  separate  from  the  "  last 
runnings  "  of  the  stills,  and  which  was  either  burnt 
or  used  as  a  lubricant,  became  greatly  in  demand 
and  its  value  was  enhanced  from  pence  to  pounds. 

The  preparation  of  alizarin  is  simple.  Anthracene 
is  first  converted  into  anthraquinone  by  heating 
with  potassium  bichromate  and  sulphuric  acid, 
and  the  anthraquinone  then  converted  into  its 
sulphonic  acid  derivative  by  heating  with  concen- 
trated sulphuric  acid.  When  this  compound  is 
fused  with  caustic  soda,  in  presence  of  a  quantity 
of  potassium  or  sodium  chlorate,  alizarin  or  di- 
hydroxy-anthraquinone  (anthraquinone  with  two 

hydroxyl  groups) 

o     OH 

)H 


o 
s    formed.     In    this    process    we    see    the    first 


ANTHRACENE  DYES  AND  VAT  DYES    83 

triumphant  success  of  the  chemist  in  the  artificial 
production  of  a  natural  colouring  matter,  and  in 
this  way  the  madder  dye  can  be  manufactured 
much  more  cheaply  than  Nature  can  produce  it. 

Alizarin  is  a  compound  which  is  insoluble  in  cold 
water,  and  is  generally  put  on  the  market  in  the 
form  of  a  paste  containing  10  or  20  per  cent,  of 
alizarin.  Over  2000  tons  of  this  dye-stuff  are  now 
manufactured  annually,  and  of  this  and  other 
anthracene  dyes  the  United  Kingdom  imported, 
in  1913,  over  3000  tons,  of  the  value  of  about 
£270,000. 

The  main  importance  of  alizarin  lies  in  its  wide- 
spread use  in  cotton  dyeing  and  printing.  Unlike 
some  of  the  azo-dyes  to  which  reference  has  been 
made,  alizarin  will  not  dye  either  vegetable  or  animal 
fibres  directly,  but  only  with  the  aid  of  mordants. 
As  mordants,  substances  are  mainly  used  which 
give  rise  to  oxides  of  metals  with  which  the  dye 
forms  an  insoluble  compound  or  lake,  and  the  colour 
produced  on  the  fibre  depends  on  the  metallic  oxide 
used.  With  alumina  as  mordant,  alizarin  gives  a 
bright  red  colour  (as  in  Turkey  red)  ;  with  oxide 
of  chromium  maroon  is  obtained ;  whereas  with 
oxide  of  iron  alizarin  produces  a  violet  shade. 
Orange  shades  can  also  be  produced  by  using  tin 
salts  as  mordants. 

Besides  alizarin,  a  large  number  of  other  deriva- 
tives of  anthraquinone  are  used  as  dyes.  By  intro- 


84        THE  TREASURES  OF  COAL  TAR 

ducing  three,  four,  five,  and  six  OH-groups  into 
the  molecule  of  anthraquinone,  one  obtains  pur- 
purin  (which  is  also  found  along  with  alizarin 
in  the  madder  root),  alizarin  bordeaux,  alizarin 
cyanine  R,  and  anthracene  blue,  which  give 
red,  violet,  and  blue  shades.  By  the  introduction 
of  the  nitro-group  (NO2)  into  alizarin,  one  obtains 
alizarin  orange  and  alizarin  brown, 

O       OH  O       OH 

H 


O  O       N0a 

Alizarin  Orange  Alizarin  Brown 

and  by  the  introduction  of  the  sulphonic  acid  group 
(SO2OH),  alizarin  red  S.  The  introduction  of  still 
other  groups  into  the  molecule  of  anthraquinone 
gives  rise  to  dyes  of  widely  varying  shades — browns, 
greens,  blues,  and  violets — the  shade  obtained 
depending  both  on  the  nature  of  the  group  intro- 
duced and  on  its  position  in  the  molecule  (cf.  p.  65). 

Closely  related  to  the  anthracene  dyes  are  the 
acridine  dyes,  some  of  which  are  used  mainly  for 
dyeing  leather  of  a  yellow  colour.  One  of  these 
dyes,  trypa-flavine  or  acri-flavine,  has  recently 
found  important  application  as  an  antiseptic  (p.  no). 

One  of  the  most  important  developments  in 
recent  years  has  been  the  production  of  a  series  of 
dyes  known  as  the  indanthrenes,  a  series  of  dye- 


ANTHRACENE  DYES  AND  VAT  DYES    85 

stuffs  derived  from  arithraquinone  and  possessing 
exceptional  fastness  to  light  and  to  cleansing  agents. 
Although  first  discovered  only  in  1901,  by  R.  Bohn 
of  the  Badische  Anilin-  und  Soda-Fabrik,  these  dyes 
are  now  manufactured  in  twelve  or  thirteen  different 
shades  covering  the  whole  range  of  colours  from 
red  to  blue.  On  account  of  their  fastness  they  are 
largely  employed  in  the  dyeing  of  Sundour  and 
other  guaranteed  "  fadeless "  fabrics.  The  first 
and  one  of  the  most  important  of  these  dyes,  in- 
danthrene  blue,  was  obtained  by  fusing  an  amino- 
derivative  of  anthraquinone, 
o 


NH2 


with  caustic  alkali,  whereby  two  molecules  of  this 
compound  were  caused  to  join  up  and  yield  indan- 
threne  blue,  thus : 


86        THE  TREASURES  OF  COAL  TAR 

Other  indanthrene  dyes,  derivatives  of  anthraquinone, 
have  also  been  obtained,  such  as,  indanthrene  yellow 
or  flavanthrene,  indanthrene  red,  indanthrene 
green,  etc.,  as  well  as  the  dyes  known  as  algol 
yellow,  algol  red,  helindon  yellow,  etc.  Until 
recently,  none  of  these  dyes  had  been  manufactured 
in  England,  but  in  the  present  year  (1917)  indan- 
threne blue  was  manufactured  by  British  Dyes,  Ltd., 
and  placed  on  the  market  under  the  name  of 
chloranthrene  blue,  and  a  second  blue  anthracene 
dye-stuff  for  wool  and  silk  has  also  been  placed  on 
the  market  under  the  name  of  alizarine  delphinol. 
Some  years  after  the  introduction  of  indanthrene 
blue  there  was  discovered,  by  a  strange  mishap, 
a  new  method  of  making  this  compound  and  others 
of  a  similar  kind.  It  befell  in  this  way.  For  the 
production  of  another  dye  derived  from  anthra- 
quinone,  a  dye  called  sky-blue  alizarine,  the  neces- 
sary ingredients  were  heated  for  some  time  in  a 
vessel  made  of  iron.  In  the  course  of  time  new 
apparatus  had  to  be  installed ;  and  with  this 
apparatus  no  sky-blue  alizarine  was  obtained,  but 
something  entirely  different.  What  could  be  the 
reason  of  the  failure  ?  The  process  was  carried 
out  in  the  same  way  as  before  and  under  the  same 
direction.  The  apparatus,  certainly,  was  new,  but 
it  was  exactly  the  same  as  the  old  apparatus.  And 
yet,  no ;  it  was  not  exactly  the  same.  The  new 
apparatus,  instead  of  being  entirely  of  iron,  had  a 


After  this  book  had  passed  through  the  press, 
the  production  of  a  number  of  anthracene  dyes 
was  announced  by  Sol  way  Dyes  Co.,  Carlisle  (an 
offshoot  of  Morton  Sundour  Fabrics,  Ltd.),  the 
production  of  such  dyes  having  been  begun,  on  a 
commercial  scale,  as  early  as  February  1915.  So 
far,  the  manufacture  of  the  following  dyes  has 
been  taken  up  by  the  above  firm,  the  Company's 
trade  name  for  the  dye  being  given  in  brackets : 
Indanthrene  yellow  G.  (Caledon  yellow) ;  indan- 
threne  blue  (Caledon  blue) ;  indanthrene  dark  blue 
B.O.  (Caledon  purple) ;  indanthrene  green  B. 
(Caledon  green) ;  indanthrene  brown  B.B.  (Caledon 
brown) ;  indanthrene  red  B.N.  (Caledon  red) ; 
indanthrene  pink  B.  (Caledon  pink) ;  indanthrene 
violet  R.  Extra  (Caledon  violet) ;  alizarine  sapph- 
irole  (Solway  blue) ;  alizarine  cyanine  green 
(Kymric  green). 


Page  86 


ANTHRACENE  DYES  AND  VAT  DYES    87 

copper  lid.  But  surely  that  could  not  be  the  cause 
of  the  different  behaviour.  Yet  so  it  was,  for  the 
small  trace  of  copper  derived  from  the  lid  exerted 
a  powerful  catalytic  influence,  as  it  is  called,  on  the 
reaction.  Merely  by  its  presence  the  copper  greatly 
accelerated  the  reaction  in  one  particular  direction, 
and  so  led  to  the  production  not  of  sky-blue  alizarine 
but  of  an  indanthrene  dye.  By  utilising  this 
property  of  copper  indanthrene  blue  and  other 
valuable  dyes,  belonging  to  this  and  similar  series, 
could  be  obtained.  To  some  the  discovery  of  this 
process  may  appear  merely  as  a  "  lucky  chance," 
but  it  must  be  remembered  that  it  is  only  he  who 
has  the  discerning  eye  and  the  understanding  mind 
who  can  turn  the  "  lucky  chance  "  to  profit.  It  is, 
however,  part  of  the  romance  of  scientific  investi- 
gation that  the  "  lucky  chance  "  is  also  one  of  the 
rewards  that  come  to  those  who  actively  and  per- 
sistently till  the  virgin  soil  of  science. 

Besides  those  already  mentioned,  several  other 
series  of  dyes  are  known  derived  from  the  constitu- 
ents of  coal-tar,  but  although  a  number  of  these 
dyes  are  of  much  importance  (e.g.  the  rhodamines, 
the  saffranines,  etc.),  a  discussion  of  them  would 
lead  beyond  the  limits  allowable.  To  one  important 
series  of  dyes,  however,  known  as  the  sulphur  or 
sulphide  dyes,  a  brief  reference  must  be  made. 
These  dyes,  which  have  only  recently  been  exten- 


88        THE  TREASURES  OF  COAL  TAR 

sively  introduced,  are  obtained  by  heating  various 
organic  materials  with  sulphur  and  sodium  sulphide, 
and  are  now  manufactured  in  large  quantity  and 
in  various  shades  of  red,  yellow,  brown,  green, 
violet,  blue,  and  black.  These  dyes  are,  at  the 
present  time,  in  much  favour  with  dyers,  for  most 
of  them  are  substantive  or  direct-dyeing  colours, 
and  possess  a  fastness  to  light  and  to  washing  at 
least  equal  to  that  of  such  a  fast  dye  as  indigo.  For 
the  production  of  these  sulphide  dyes  aromatic 
amino-compounds  and  nitro-derivatives  of  the 
phenols  are  most  suitable  for  use,  but  a  number 
have  also  been  obtained  from  anthraquinone. 
Three  of  these  sulphur  dyes,  Khaki  yellow  C,  Khaki 
Brown  C,  and  Cross  Dye  Black  F.N.G.,  are  largely 
used  at  the  present  time  for  the  production  of  khaki 
colour  on  cotton. 

The  series  of  dyes  to  which  reference  has  just 
been  made,  the  sulphur  dyes  as  also  the  indanthrene 
dyes  derived  from  anthraquinone,  belong  to  what 
are  called  by  dyers,  vat-dyes.  Owing  to  the  insolu- 
bility of  these  dyes  in  water,  it  is  not  possible  to 
prepare  dye-baths  in  the  ordinary  manner ;  and 
for  the  purpose  of  dyeing  a  less  direct  method  must 
be  employed.  In  using  these  dyes  advantage  is 
taken  of  the  fact  that  they  are  comparatively  readily 
reduced  to  compounds  (so-called  leuco-compounds), 
which  are  soluble  in  alkalies.  The  material  to  be 


ANTHRACENE  DYES  AND  VAT  DYES    89 

dyed  is  therefore  dipped  in  the  alkaline  solution 
of  the  leuco-compound — now  generally  produced 
from  the  dye  by  reduction  with  sodium  hydro- 
sulphite — which  is  readily  taken  up  both  by  animal 
and  vegetable  fibres.  On  exposing  the  material 
to  the  air,  the  original  dye-stuff  is  produced  in  the 
fibre  in  an  exceedingly  fast  form,  owing  to  the 
oxidation  of  the  leuco-compound  by  the  atmo- 
spheric oxygen.  Sometimes  the  leuco-compound 
of  the  dye  is  colourless  or  very  faintly  coloured, 
but  in  other  cases,  e.g.  in  the  case  of  the  indanthrene 
dyes,  the  leuco-compound  may  have  a  very  marked 
colour  which  is  generally  different  from  that  of  the 
original  dye-stuff.  Thus,  indanthrene  yellow  or 
flavanthrene  yields  a  blue  leuco-compound  which, 
on  exposure  to  the  air,  changes  into  a  fast  yellow 
dye.  Similarly,  indanthrene  red  and  indanthrene 
green  yield  purple  and  blue  reduction  compounds 
respectively,  which  are  taken  up  by  the  fabric 
from  the  bath  and  which  then,  on  exposure  to  the 
air,  change  to  red  and  green.  Although,  formerly, 
vat-dyeing  was  a  somewhat  difficult  and  uncertain 
process,  it  has  now  been  rendered  as  easy  and  as 
simple  as  dyeing  from  the  ordinary  dye-bath. 
That  this  is  so  is  due  largely  to  the  careful  investiga- 
tion of  the  process  consequent  on  the  successful 
artificial  production  of  by  far  the  most  important 
of  the  vat-dyes,  indigo,  a  discussion  of  which  is 
reserved  for  the  following  chapter. 


CHAPTER  VIII 

INDIGO  AND   ITS   DERIVATIVES 

OF  all  the  dyes  now  in  use  none  equals  in  commercial 
importance  or  has  aroused  such  general  interest  as 
indigo,  not  only  owing  to  the  fact  that  its  production 
from  coal-tar  hydrocarbons  constitutes  one  of  the 
greatest  achievements  of  pure  and  applied  chemistry, 
but  also  by  reason  cf  the  enormous  economic  con- 
sequences of  that  success.  Known  from  a  very 
remote  period,  indigo  was,  until  about  twenty  years 
ago,  obtained  solely  from  certain  species  of  plants, 
the  Indigoferce,  cultivated  more  especially  in  India, 
China,  and  Egypt.  Even  in  Europe,  as  late  as  the 
seventeenth  or  eighteenth  century,  an  indigo-bear- 
ing plant,  the  woad  (I satis  tinctoria),  was  cultivated 
to  no  small  extent,  and  its  cultivation  still  lingers 
on  in  the  eastern  counties  of  England.  In  the 
sixteenth  century,  with  the  opening  up  of  trade 
with  the  East,  the  superior  Indian  indigo  began 
to  make  its  appearance  in  Europe,  but  for  many 
years,  owing  to  the  influence  of  those  interested  in 
the  growing  of  woad,  its  introduction  met  with  a 
powerful  opposition,  and  the  use  of  the  "  devilish 
drug,"  as  it  was  called,  was  prohibited  by  law.  In 

90 


INDIGO  AND  ITS  DERIVATIVES         91 

the  eighteenth  century,  however,  this  ban  was 
removed  and  the  use  of  Indian  indigo  gradually 
extended  over  the  whole  of  Europe.  As  a  con- 
sequence, the  Indian  indigo  plantations  came  to 
control  the  markets  of  the  world. 

Indigo  does  not  occur  as  such  in  the  plant,  but  as 
a  glucoside,  called  indican,  which  is  found  almost 
exclusively  in  the  leaves.  To  obtain  the  indigo, 
the  cut  plant  is  placed  in  steeping  vats  and  covered 
with  water.  Owing  to  the  presence  of  an  enzyme 
(or  ferment)  in  the  leaves,  fermentation  takes  place 
and  the  indican  undergoes  decomposition  into 
glucose  and  the  leuco-compound  (p.  88)  of  indigo. 
On  agitating  the  resulting  solution  with  air,  this 
leuco-compound  is  oxidised  with  production  of 
indigo,  which  separates  out  as  an  insoluble  powder. 
Although  indigo-blue  or  indigotin  is  the  main  con- 
stituent, the  natural  indigo  also  contains  varying 
amounts  of  other  compounds,  indirubin  or  indigo 
red,  indigo  brown,  indigo  yellow,  and  indigo  gluten. 

The  industry  was  a  large  and  lucrative  one.  In 
1896-7  the  area  under  cultivation  amounted  to 
1,583,808  acres,  and  the  weight  of  indigo  produced 
was  8433  tons,  the  value  of  which  amounted  to 
about  £4,000,000.  It  was  a  rich  prize,  therefore, 
which  the  large  German  chemical  manufacturers 
saw  before  them  when  they  set  themselves  earnestly 
to  capture  the  indigo  market  by  producing  the 
dye  artificially.  The  fight  was  a  long  one,  for 


92        THE  TREASURES  OF  COAL  TAR 

seventeen  years  the  struggle  went  on,  and  close 
on  £1,000,000  was  spent  on  the  campaign,  but  in 
the  end  the  genius  and  resourcefulness  of  the 
chemist,  the  persistence  and  enterprise  of  the  direc- 
tors of  German  chemical  industry — themselves  expert 
chemists — won  the  day ;  and  in  October  1897  syn- 
thetic indigo  was  placed  on  the  market  in  com- 
petition with  the  product  from  the  Indian  planta- 
tions. And  what,  to-day,  is  the  result  of  the  com- 
petition ?  Since  1896-7  the  area  under  cultivation 
for  indigo  fell  from  1,583,808  acres  to  214,500  acres 
in  1912-13 ;  and  whereas,  in  1896,  India  exported 
indigo  to  the  value  of  over  £3,500,000,  in  1913  her 
export  was  only  worth  about  £60,000.  In  1913, 
on  the  other  hand,  Germany  exported  nearly  6700 
tons  of  pure  synthetic  indigo  (indigotin  or  indigo- 
blue),  valued  at  about  £2,750,000.  In  the  above 
period,  moreover,  the  price  of  pure  indigo  was  about 
halved. 

Whether  the  decline  of  the  Indian  indigo  plan- 
tations will  continue  cannot  be  foreseen.  Until 
recently,  the  processes  employed  in  recovering  the 
indigo  were  crude  and  unscientific,  but  in  recent 
years  many  improvements  have  been  effected,  and 
new  species  of  plants,  producing  a  larger  proportion 
of  indigo,  have  been  introduced.  Further  improve- 
ments in  this  direction  may  still  be  possible,  and  as 
many  dyers  still  feel  a  preference  for  the  natural 
product,  for  securing  certain  effects  at  least,  it  is 


INDIGO  AND  ITS  DERIVATIVES        93 

possible  that  the  Indian  production  of  natural 
indigo  may  still  be  maintained.  A  considerable 
change  has,  however,  already  been  produced  in 
Indian  agriculture,  and  many  acres  of  land  formerly 
under  cultivation  for  indigo  have  been  made  avail- 
able for  the  growth  of  cotton  or  of  food-stuffs. 

As  far  back  as  1880  the  artificial  production  of 
indigo  was  first  achieved  by  the  German  chemist 
Adolf  von  Baeyer,  who  used  as  his  raw  material 
the  coal-tar  hydrocarbon  toluene.  The  patent 
of  this  process  was  acquired  jointly  by  the  two 
largest  dye-manufacturing  firms  in  Germany,  the 
Badische  Anilin-  und  Soda-Fabrik  of  Ludwigshaven, 
and  Meister,  Lucius  &  Briining  of  Hoechst.  But 
although  the  laboratory  production  of  indigo  con- 
stituted an  achievement  of  the  highest  scientific 
importance,  its  commercial  development  proved 
to  be  impracticable.  Indigo  could,  of  course,  be 
manufactured,  and  manufactured  in  quantity,  but 
not  at  a  price  which  would  allow  the  artificial  to 
compete  with  the  natural  dye.  Moreover,  the  raw 
material,  toluene,  was  not  at  that  time  procurable 
in  sufficient  amount  to  make  the  complete  displace- 
ment of  the  natural  indigo  possible. 

Ten  years  later,  in  1890,  a  new  method  of  syn- 
thesising  indigo  was  discovered  by  Heumann,  and 
this  method  was  subsequently  developed  along 
two  different  lines  into  commercially  successful 


94        THE  TREASURES  OF  COAL  TAR 


processes.  Both  methods  involve  a  considerable 
number  of  distinct  reactions  and  require  the  use  of 
a  number  of  different  substances,  of  which  sulphuric 
acid,  ammonia,  chlorine,  acetic  acid,  and  sodium 
are  the  chief ;  and  the  success  of  the  synthesis  as 
a  whole  depends  on  the  success  with  which  each 
step  of  the  process  can  be  carried  out,  and  on  the 
cost  of  the  substances  employed.  We  shall  now 
see  how  the  great  difficulties  involved  were  overcome. 
In  the  process  worked  at  Ludwigshaven  the  start- 
ing-point in  the  synthesis  is  naphthalene,  one  of 
the  most  abundant  constituents  of  coal-tar ;  and 
the  various  steps  of  the  process  can  be  represented, 
graphically,  thus  : 

A/c°\ 


COOH 


/co\ 


Naphtha- 
lene 


COOH 
Phthalic 
acid 


NH2 


€OOH 
Anthranilic 
acid 


H2— COOH 

XOOH 

Phenyl-glycine-ortho- 
carboxylic  acid 

NH  NH 


Indoxyl 


(7) 


CO 

Indigotin 


INDIGO  AND  ITS  DERIVATIVES         95 

(i)  Naphthalene  is  converted  into  phthalic  acid 
by  heating  with  fuming  sulphuric  acid  ;  (2)  phthalic 
acid,  on  being  heated,  passes  into  phthalic  anhy- 
dride ;  (3)  phthalic  anhydride,  on  being  heated  with 
ammonia,  yields  phthalimide  which  (4)  on  being 
treated  with  bleaching  powder  or  with  sodium 
hypochlorite,  forms  anthranilic  acid.  By  the  action 
of  monochloracetic  acid  on  the  latter,  (5),  phenyl- 
glycine-ortho-carboxylic  acid  is  produced,  and  (6) 
this  compound,  on  being  fused  with  caustic  soda, 
passes  into  indoxyl ;  (7)  and  on  oxidising  this 
substance  with  atmospheric  oxygen,  indigo-blue 
or  indigotin  is  formed.  On  attempting  to  carry 
out  this  series  of  operations  on  a  large  scale,  it  was 
found  that  all  the  steps,  except  the  first,  could  be 
carried  out  in  a  commercially  successful  manner. 
In  the  case  of  the  first  stage  of  the  process,  however, 
it  was  found  that  the  conversion  of  naphthalene 
into  phthalic  acid  did  not  proceed  sufficiently  readily, 
and  the  cost  involved  was  so  great  that  it  rendered 
the  industrial  production  of  indigo  unremunerative. 
A  small  obstacle,  apparently,  but  a  very  effective 
one  !  While  engaged  in  an  endeavour  to  overcome 
this  difficulty,  a  fortunate  mischance  came  to  the 
assistance  of  the  manufacturer,  for,  through  the 
accidental  breaking  of  a  thermometer  immersed 
in  the  heated  mixture  of  naphthalene  and  sulphuric 
acid,  it  was  discovered  that  mercury  acts  as  an 
efficient  catalyst  in  the  conversion  of  naphthalene 


96        THE  TREASURES  OF  COAL  TAR 

into  phthalic  acid,  and  facilitates  the  process  to 
such  a  degree  as  to  allow  it  to  be  carried  out  with 
commercial  success.  It  was,  in  fact,  this  fortunate 
discovery  that  first  ensured  the  success  of  the 
synthetic  production  of  indigo. 

Another  process,  likewise  based  on  the  work  of 
Heumann,  has  also  been  successfully  developed, 
and  has  been  employed  for  many  years  by  the  firm 
of  Meister,  Lucius  &  Briining.  In  this  case  benzene 
forms  the  starting-point.  In  the  manner  already 
described  (p.  54),  benzene  is  converted  first  into 
nitro-benzene  and  then  into  aniline.  By  the  action 
of  monochloracetic  acid  on  aniline,  phenyl-glycine 
is  produced,  and  when  this  is  fused  with  sodamide, 
indoxyl  is  formed  and  can  then  be  converted  into 
indigo  tin  by  oxidation,  as  in  the  previous  process.1 
The  steps  of  this  process  can  be  represented  thus  : 

C6H6  — >  C6H5'N02  — >  C6H5-NH2  -->  C6H5-NH'CH2-COOH 
Benzene  Nitro-  Aniline  Phenyl-glycine 

benzene 

/NHv  /NH\  /NH\ 

->C6H/         >CH2  ->  C6H/         >C  =  C<          >C6H4 
\CO  /  \CO  /  \CO  / 

Indoxyl  Indigotin 

1  In  August  1916  the  indigo  factory  at  Ellesmere  Port,  formerly 
belonging  to  the  German  firm,  Meister,  Lucius  &  Briining,  was 
transferred  to  Messrs  Levinstein,  Ltd.,  of  Blackley,  Manchester, 
and  by  the  end  of  that  year  British-made  synthetic  indigo  was 
placed  on  the  market.  Although,  owing  to  the  exigencies  of 
the  war,  chloracetic  acid  was  commandeered  by  the  British 
Government,  a  method  of  producing  phenyl-glycine  without  the 
use  of  that  compound  was  successfully  worked  out  by  the 
chemists  of  the  British  company,  and  an  adequate  supply  of 
British-made  indigo  (known  as  Indigo  LL)  is  now  available. 


INDIGO  AND  ITS  DERIVATIVES         97 

The  commercial  success  of  the  production  of 
indigo  depended,  however,  not  only  on  the  success 
with  which  the  different  steps  in  the  process  could 
be  carried  out,  but  also  on  the  production  of  the 
necessary  reagents  at  a  sufficiently  low  cost.  In 
the  first  process  the  conversion  of  naphthalene 
requires  a  fuming  sulphuric  acid  of  a  much  greater 
concentration  than  the  acid  produced  by  the  old 
leaden-chamber  process ;  and,  moreover,  large 
quantities  of  sulphur  dioxide  were  formed  during 
the  reaction,  the  recovery  of  which  in  an  advan- 
tageous manner  was  an  essential  condition  of  suc- 
cess. These  requirements,  therefore,  led  to  the 
development  of  the  so-called  "  contact  process," 
in  which  a  mixture  of  sulphur  dioxide  and  air  is 
passed  over  heated  platinised  asbestos.  The  sulphur 
dioxide  combines  with  the  oxygen  of  the  air  to  form 
sulphur  trioxide — which  unites  with  water  to  form 
sulphuric  acid — and  the  trioxide  is  passed  into 
concentrated  sulphuric  acid.  In  this  way  fuming 
sulphuric  acid,  or  "  oleum "  as  it  is  technically 
called,  is  produced.  From  this  description  the  pro- 
cess doubtless  appears  to  be  a  very  simple  one, 
but  on  attempting  to  employ  it  for  the  industrial 
production  of  fuming  sulphuric  acid,  a  difficulty 
was  met  with  which  seemed  at  first  to  be  insur- 
mountable. On  passing  the  mixture  of  sulphur 
dioxide  and  air  over  the  platinised  asbestos  all  went 
well  for  a  time  ;  but  soon  the  reaction  stopped  and 
G 


98        THE  TREASURES  OF  COAL  TAR 

no  more  sulphur  trioxide  was  formed.  After  a 
considerable  amount  of  investigation  the  cause  of 
this  behaviour  was  traced  to  the  presence  of  minute 
quantities  of  arsenic  in  the  sulphur  dioxide,  but  it 
still  required  some  years'  further  work  before  a 
successful  method  of  removing  this  arsenic  was 
discovered.  For  the  production  of  chlorine,  also, 
of  which  enormous  quantities  were  required  for  the 
manufacture  of  hypochlorite  and  of  monochlor- 
acetic  acid,  the  old  method  of  obtaining  the  gas 
from  hydrochloric  acid  was  useless,  and  a  new 
method  had  to  be  introduced,  namely,  by  passing 
a  current  of  electricity  through  a  solution  of  common 
salt  or  of  potassium  chloride,  the  chlorine  being 
then  obtained  in  a  pure  state  by  liquefaction.  In 
this  process,  also,  caustic  soda  and  hydrogen  are 
produced ;  the  former  of  these  is  required  for  the 
conversion  of  phenyl-glycine-ortho-carboxylic  acid 
into  indoxyl,  and  the  latter  is  now  available  for  the 
production  of  ammonia  (also  used  in  the  indigo 
synthesis),  by  direct  combination  with  the  nitrogen 
of  the  air.  The  acetic  acid,  of  which  3000  tons 
are  used  annually  in  the  manufacture  of  indigo,  is 
obtained  by  the  distillation  of  150,000  cubic  yards 
of  wood.  The  whole  most  impressive  story  of 
the  development  of  the  manufacture  of  synthetic 
indigo  is  one  of  unshakable  faith  in  science,  of 
chemical  and  engineering  ability  and  resourceful- 
ness, and  of  untiring  perseverance. 


INDIGO  AND  ITS  DERIVATIVES         99 

Closely  related,  chemically,  with  indigo  is  that 
other  ancient  dye,  Tyrian  purple.  Some  years  ago 
the  nature  of  this  dye  was  investigated  by  a  German 
chemist,  Friedlander,  who  extracted  it  from  the 
glands  of  two  species  of  marine  snail,  the  Murex 
brandaris  and  the  Murex  trunculus,  and  ascertained 
that  this  most  valuable  of  all  the  ancient  dyes 
is  a  derivative  of  indigo  in  which  two  atoms  of 
hydrogen  are  replaced  by  bromine  ;  and  this  dye, 
for  which,  however,  there  is  now  no  demand,  can 
be  prepared,  artificially,  with  comparative  ease. 
Other  chlorine  and  bromine  derivatives  of  indigo 
are  also  known,  and  some  are  used  as  dyes  under 
the  name  of  Ciba  dyes. 


CHAPTER  IX 

DRUGS,  PERFUMES,  AND  PHOTOGRAPHIC  DEVELOPERS 

IN  the  sixteenth  and  seventeenth  centuries,  following 
on  the  long  period  of  alchemistic  activity  and  the 
somewhat  sterile  search  for  the  "  philosopher's 
stone,"  chemistry,  under  the  influence  of  Para- 
celsus, found  its  main  glory  in  acting  as  the  hand- 
maid of  medicine,  and  its  chief  task  in  the  prepara- 
tion of  drugs  and  in  the  study  of  their  action  on 
the  human  organism.  But  the  efforts  of  those 
early  medico-chemists,  or  iatro-chemists  as  they 
have  been  called,  have  been  completely  eclipsed 
by  the  brilliant  discoveries  of  the  modern  organic 
chemist,  who  has  made  available  for  use  a  large 
array  of  new  drugs  and  medicinal  preparations. 
Since  many  of  the  most  important  of  these  sub- 
stances are  prepared  from  the  constituents  of  coal 
tar,  it  will  readily  be  understood  that  this  branch 
of  chemical  industry — as  indeed  the  whole  domain 
of  the  so-called  "  fine  "  (organic)  chemicals — has 
been  developed  mainly  in  Germany,  and  this  largely 
as  an  offshoot  or  companion  industry  of  the  manu- 
facture of  artificial  colouring  matters.  This  is 
accounted  for  partly  by  the  fact  that  in  many  cases 
100 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  101 

the  raw  materials  of  manufacture  are  the  same,  and 
that  many  of  the  reagents  and  coal-tar  "  inter- 
mediates," required  for  the  manufacture  of  dyes, 
serve  also  for  the  manufacture  of  drugs,  perfumes, 
and  other  organic  chemicals.  But  a  further  reason 
is  to  be  found  in  the  greater  encouragement  given 
to  the  study  of  chemistry  and  to  chemical  research, 
which  has  made  possible  the  extraordinary  achieve- 
ments in  the  domain  of  synthetic  dyes  and  drugs. 

Although  the  production  of  synthetic  drugs  may 
be  said  to  date  from  the  discovery  of  chloroform 
and  of  chloral  by  Liebig  in  1832 — and  who  will  be 
so  bold  as  to  assess  the  value  of  these  discoveries 
to  mankind? — it  was  not  till  1881  that  the  first 
drug  derived  from  the  constituents  of  coal  tar  was 
prepared.  In  that  year  were  discovered  kairine 
and  other  antipyretic  derivatives  of  quinoline,  a 
compound  which  is  produced  by  heating  aniline 
with  a  mixture  of  sulphuric  acid  and  glycerin,  in 
presence  of  nitrobenzene.  These  antipyretics  had, 
however,  but  small  success.  In  1883  Ludwig  Knorr 
prepared  the  important  febrifuge,  antipyrine  or 
phenazone,  of  which  very  large  quantities  were 
at  one  time  consumed.  The  commercial  success, 
indeed,  of  this  drug  was  so  great  that,  before  the 
expiration  of  the  patent,  the  profits  in  one  year  are 
stated  to  have  amounted  to  no  less  than  £60,000. 
In  the  preparation  of  this  compound  there  is  used 
a  substance  known  as  phenyl-hydrazine  which  is 


102   THE  TREASURES  OF  COAL  TAR 

prepared  from  aniline,  and  this  in  turn  from  benzene. 
The  investigation  of  the  physiological  action  of 
antipyrine,  undertaken  on  account  of  its  supposed 
chemical  relationship  with  the  alkaloid  quinine, 
led  to  the  discovery  of  its  valuable  antipyretic 
properties.  It  was  the  first  of  a  series  of  synthetic 
antipyretics  which  have,  with  a  certain  amount  of 
success,  entered  into  competition  with  and  partially 
displaced  the  natural  alkaloid  quinine.  It  may, 
however,  be  said  that  valuable  as  these  drugs  have 
proved  to  be,  they  are  drugs  which  combat  the 
symptoms  of  disease  and  not  the  disease  itself, 
and  they  do  not  possess  the  specific  curative  pro- 
perties shown,  for  example,  by  quinine  in  relation 
to  malaria. 

A  derivative  of  antipyrine,  known  as  pyramidone, 
has  also  been  introduced  as  an  antipyretic.  It  is 
more  powerful  than  antipyrine,  and  has  been  found 
to  possess  certain  advantages  over  the  latter,  more 
especially  in  not  exercising  an  injurious  influence 
on  the  heart. 

In  1887  antipyrine  met  with  a  powerful  com- 
petitor, antifebrine,  and  it  is  to  the  discovery 
of  this  substance,  more  especially,  that  the  great 
development  which  has  taken  place  in  recent  times 
in  the  industrial  production  of  synthetic  drugs,  is 
due.  The  discovery  of  the  antipyretic  properties 
of  antifebrine  was  due  to  a  mistake  on  the  part  of 
a  laboratory  boy  who  supplied  this  substance  in 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  103 

place  of  naphthalene.  During  a  pharmacological 
investigation  of  the  substance  its  strongly  anti- 
febrile action  was  detected,  and  from  a  chemical 
analysis  it  was  learned  what  the  substance  really 
was. 

Antifebrine  is  the  trade  name  for  the  compound 
known  in  chemistry  as  acetanilide,  which,  as  is 
shown  by  the  formula,  CH3-CO-NH-C6H5,  is  formed 
by  the  combination  of  acetic  acid,  CH3-CO-OH, 
with  aniline,  NH2-C6H5,  with  the  elimination  of 
the  OH-group  from  acetic  acid  and  a  hydrogen 
atom  from  aniline.  Mixed  with  bicarbonate  of 
soda,  acetanilide  has  also  been  sold  as  a  "  head- 
ache powder." 

The  discovery  of  the  physiological  action  of  anti- 
febrine,  and  the  circumstances,  more  especially, 
under  which  that  discovery  was  made,  greatly 
stimulated  the  investigation  of  the  physiological 
action  of  other  substances.  As  a  result  of  these 
investigations,  interesting  relationships  between 
physiological  action  and  chemical  constitution 
became  known.  Thus,  the  physiological  action  is 
found,  in  many  cases,  to  be  due  to  the  presence 
of  certain  groupings  of  atoms  in  the  molecule,  and 
can  be  modified  or  even  entirely  altered  by  the 
introduction  of  different  groups  into  the  molecule. 
Thus,  aniline  itself  is  a  powerful  febrifuge,  but  at 
the  same  time  it  is  highly  poisonous,  owing  to  its 
destructive  action  on  the  red  blood  corpuscles. 


104      THE  TREASURES  OF  COAL  TAR 

By  introducing  the  group  CH3-CO  (aeetyl),  the  com- 
pound is  rendered  more  stable  and  the  toxicity  is, 
in  consequence,  reduced. 

Similar  considerations  led  also  to  the  production, 
in  1887,  of  another  antipyretic  and  antineuralgic 

/0-C2H6 
drug,  phenacetine,  C6H4<^  ,  the  derivation 

XNH-CO'CH3 

/OH 

of  which  from  C8H4<^  (para-amino-phenol)   is 

XNH2 

obvious.  This  compound  is  prepared  from  phenol 
in  the  same  way  as  aniline  is  prepared  from  benzene 
(p.  54),  by  converting  phenol  into  a  nitro-phenol, 

/OH 

CeH4\       ,    and    then    converting    the    nitro-group 

XNO2 

into  the  amino-group  by  means  of  iron  and  hydro- 
chloric acid.  Phenacetine  is  the  most  important 
and  most  largely  used  of  all  the  synthetic  antipyretic 
and  analgesic  drugs,  over  eight  tons  of  this  com- 
pound being  imported  into  Great  Britain  in  1909. 

Valuable  medicinal  preparations  have  also  been 
derived  in  recent  years  from  salicylic  acid.    This 

/OH 

compound,    C6H4<^          ,     prepared    from     phenol 
XOOH 

with  the  help  of  carbon  dioxide  or  carbonic  acid 
gas,  possesses  anti-neuralgic  and  anti-rheumatic 
properties,  but  its  use  gives  rise  to  disorders  of  the 
digestion.  By  the  introduction  of  the  acetyl-group 
(CH3-CO),  however,  one  obtains  the  compound, 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  105 


acetyl-salicylic  acid,  C6H4<  ,   which,    under 


the  name  of  aspirin,  has  come  to  be  recognised  as 
one  of  the  most  valuable  of  the  anti-neuralgic  and 
anti-rheumatic  drugs.  Discovered  in  1899,  and 
formerly  manufactured  only  in  Germany,  it  is 
now  produced  by  several  firms  in  England  and  is 
sold  under  the  registered  trade  names  of  empirin 
(Burroughs  Wellcome  &  Co.),  regepyrin  (Boots), 
etc. 

Other  derivatives  of  salicylic  acid  are  employed 
as  intestinal  antiseptics.  Thus,  by  the  introduc- 
tion of  the  phenyl  group  into  salicylic  acid,  there  is 

/OH 

produced  the   compound  salol,  C6u/  ,   a 

Ncoo-c6H5 

valuable  intestinal  antiseptic.  It  is  readily  prepared 
from  phenol  and  salicylic  acid,  or  even  by  heating 
salicylic  acid  alone.  Other  derivatives  of  a  similar 
nature  can  also  be  prepared  and  find  a  similar 
application.  These  compounds,  although  not  acted 
on  by  the  stomach  juices,  are  broken  up  by  the 
alkaline  secretions  in  the  intestine,  with  produc- 
tion of  salicylic  acid.  This  compound  then  produces 
partial  asepsis  by  restricting  the  development  of 
bacteria  and  undue  fermentative  action  in  the 
alimentary  canal. 

With  regard  to  general  antiseptics,  we  have 
already  seen  (p.  25)  that  phenol  (carbolic  acid) 


io6      THE  TREASURES  OF  COAL  TAR 

and;  in  a  still  greater  degree,  the  cresols,  have  a 
powerful  bactericidal  action.  The  cresols,  more- 
over, have  the  advantage  over  phenol  in  being  less 
toxic  to  the  organism.  By  the  introduction  of 
bromine  into  the  molecule  of  phenol  or  cresol,  the 
bactericidal  action  is  greatly  increased,  so  that 
pentabrom-phenol  (phenol  with  five  bromine  atoms), 
for  example,  is  about  five  hundred  times  as  effective 
as  phenol.  Similarly,  tetrabrom-ortho-cresol  (ortho- 
cresol  with  four  atoms  of  bromine)  is  a  valuable 
antiseptic  which  is  almost  non-toxic,  but  which, 
even  in  a  dilution  of  only  i  part  in  200,000,  will 
destroy  diphtheria  bacilli.  It  is,  in  this  respect, 
250  times  as  effective  as  phenol. 

In  1832,  as  we  have  already  seen,  Liebig  dis- 
covered the  compound  chloroform,  the  introduction 
of  which  as  an  anaesthetic  by  Sir  James  Simpson, 
in  1847,  marked  the  beginning  of  a  new  era  in 
operative  surgery.  But  neither  this  nor  any  of 
the  other  general  anaesthetics  now  employed  are 
derived  from  coal  tar.  In  recent  years,  however,  a 
number  of  valuable  local  anaesthetics,  derived  from 
the  constituents  of  coal  tar,  have  been  prepared 
and  introduced  as  substitutes  for  the  naturally 
occurring  alkaloid  cocaine.  Thus,  anaesthesine, 

NH2<^     \co-oc8H8,    is  an  important  local  anaes- 
thetic prepared  from  benzoic  acid,  which  is,  in  turn, 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  107 

prepared  indirectly  from  toluene,  and  novocaine 
is  a  similar  compound  of  rather  more  complex 
constitution, 

NH2/~     ~\CO-OCH2-CH2'N(C2H6)2,HC1 

Stovaine,  alypine,  and  beta-eucaine  are  also 
valuable  local  anaesthetics  (the  last-mentioned  is 
also  used  in  the  treatment  of  sciatica  and  neuralgia) 
in  the  preparation  of  which  coal-tar  products  play 
a  part. 

Some  of  these  anaesthetics  are  frequently  used 
along  with  another  compound,  adrenaline,  which, 
although  not  an  anaesthetic,  has  powerful  physio- 
logical properties.  When  adrenaline,  the  active 
principle  of  the  supra-renal  glands,  is  injected 
subcutaneously  or  even  applied  externally  to 
the  skin,  it  produces  a  violent  contraction  of  the 
arteries,  with  the  result  that  the  blood  pressure 
rapidly  rises,  the  blood  is  driven  away  from  the 
injected  tissues,  and  "  bloodless  "  surgery  becomes 
a  possibility. 

Adrenaline  was  isolated  for  the  first  time  by  a 
Japanese  chemist,  Takamine,  in  1901,  from  the 
supra-renal  glands  of  sheep  and  oxen,  close  on 
1000  Ibs.  of  tissue  (representing  the  glands  from 
20,000  oxen)  being  required  to  yield  I  Ib.  of  adrena- 
line. Within  a  few  years,  however,  the  chemical 
nature  of  adrenaline  had  been  ascertained,  and  a 
process  for  preparing  it  on  an  industrial  scale  was 


io8      THE  TREASURES  OF  COAL  TAR 

worked  out  in  the  laboratories  of  the  great  dye- 
manufacturing  firm  of  Meister,  Lucius  &  Briining, 
in  Germany.  It  is  now  placed  on  the  market  under 
the  name  of  suprarenine.  It  is  a  derivative  of  the 
OH 

MOH 
,  which,  although  usually 


prepared  from  guaiacol,  a  constituent  of  beech- 
wood  tar,  may  also  be  prepared  from  phenol  or 
carbolic  acid. 

It  has  already  been  pointed  out  how  the  industry 
of  synthetic  drugs  is  closely  related  to  that  of 
synthetic  dyes ;  and  this  relationship  has  become 
a  still  closer  one  in  recent  years  owing  to  the 
important  discovery  of  "  dye  drugs "  which  we 
owe  mainly  to  the  brilliant  investigations  of  Paul 
Ehiiich  in  Germany.  The  synthetic  substitutes  for 
quinine  are,  as  we  have  seen,  merely  symptomatic 
drugs,  but  the  work  of  Ehrlich  opened  up  a  new 
field  and  led  to  the  discovery  of  drugs  which 
exercise  specific  curative  properties. 

Guided  by  the  principle  that  a  drug  acts  only  on 
organisms  by  which  it  is  absorbed,  Ehrlich  studied 
the  effect  of  various  dyes  on  different  tissues  and 
cells,  and  showed  that  certain  dyes  will  "  stain  " 
certain  tissues  but  leave  others  unstained,  just  as 
certain  colouring  matters  will  dye  wool  but  not 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS   109 

cotton.     Thus  the  dye,  methylene  blue,  is  absorbed 
by  and  stains  only  the  living  nerve,  so  that  when 
the  dye  is  injected  into  a  living  animal  the  nerve 
tissues,   but   not   the   surrounding   structures,    are 
stained.     Similarly,  different  bacteria  can  be  dis- 
tinguished by  their  selective   absorption   of  dyes. 
This    property    of    selective    absorption    has    been 
turned  to  use  with  especial  success  in  the  treatment 
of  diseases  due  to  protozoal  parasites,  because  it 
becomes   possible   to   introduce   into   an   organism 
substances  which  are  poisons  for  the  parasites  but 
are  not  absorbed  by  and  are  therefore  not  harm- 
ful   to    the    cells    of    the   organism  itself.     Thus, 
investigation  showed  that  certain  azo-dyes  of  the 
type  of   Congo  red  are   poisons  to  trypanosomes, 
the    trypanosome    of    the    South   American    horse 
disease,   "  mal   de   caderas,"    being    destroyed    by 
the  dye  trypan  red,  and  the  trypanosome  of  the 
cattle  disease,  "  piroplasmosis,"  by  another  azo-dye, 
trypan  blue,  derived  from  tolidine  and  naphthalene. 
These  dyes  are  therefore  specific  curative  agents 
for   these    diseases.      Similarly,  atoxyl    (arsamin 
or    soamin),    the    sodium    salt    of    a    compound 
obtained  by  heating  arsenic   acid  with  excess   of 
aniline,  has  the  property,  when  injected  into  the 
body,  of  killing  the  parasite  Trypanosoma  gambiense, 
which  causes  the  disease  of  "  sleeping  sickness." 

Still  more  important  is  the  success  with  which 
the  property  of  selective  absorption  has  been  utilised 


no      THE  TREASURES  OF  COAL  TAR 

in  finding  a  cure  for  the  disease  syphilis,  which  is 
due  to  a  micro-organism,  the  Spirochcete  pallida. 
It  has  been  known  from  the  time  of  Paracelsus 
that  mercury  is  a  specific  against  this  disease,  but 
mercury  is  harmful  also  to  the  human  organism. 
The  problem,  therefore,  which  Ehrlich  set  himself 
was  to  prepare  a  compound  which  would  contain 
a  toxic  material  and  which  would  be  absorbed  by 
the  germ  of  the  disease  but  not  by  the  human 
organism.  After  many  trials  and  many  failures  he 
prepared  the  now  well-known  remedy  salvarsan,  or 
as  it  is  also  called  "  606,"  which  is  the  serial  number 
of  the  compound  in  Ehrlich's  record  of  preparations. 
This  compound,  now  manufactured  in  England  by 
Burroughs  Wellcome  &  Co.,  under  the  name  khar- 
sivan,  is  a  benzene  derivative  similar  in  structure  to 
an  azo-dye  but  containing  two  arsenic  (As)  atoms  in 
place  of  the  azo-group  ( — N  :  N — ),  thus  : 

HO  / \— As     =     As— /~    ~\OH 


I  I 

NH2,HC1  NH2,HC1 

As  one  of  the  most  recent  examples  of  "  chemo- 
therapy "  based  on  selective  absorption,  there  may 
be  mentioned  the  discovery  by  Dr  Browning,  of  the 
Bland- Sutton  Institute  of  Pathology  in  London, 
that  the  yellow  coal-tar  dye,  trypa-flavine  or 
acri-flavine — a  derivative  of  a  compound  known 
as  acridine — has  the  most  valuable  property  that 
while  it  destroys  the  germs  of  blood-poisoning  it 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  in 


does  not  interfere  with  the  white  "  warrior  cells  " 
of  the  blood,  which  are  the  natural  defence  of  the 
patient  against  the  septic  organisms.  It  is,  there- 
fore, an  ideal  antiseptic  as  compared  with  the 
ordinary  antiseptics  which  destroy  with  equal 
impartiality  the  pathogenic  organisms  and  their 
natural  foes,  the  white  "  warrior  cells." 

Owing  to  the  practical  monopoly  enjoyed  by 
Germany  in  the  manufacture  of  synthetic  drugs, 
this  country  was  placed,  by  the  outbreak  of  war, 
in  a  position  of  great  gravity  as  a  result  of  the 
cutting  off  of  German  supplies  ;  and  the  shortage 
of  drugs  which  was  thereby  produced  was  clearly 
reflected  in  a  great  increase  in  price,  as  shown  in 
the  following  table  : 

EFFECT  OF  THE  WAR  ON  THE  PRICE  OF 
SYNTHETIC  DRUGS 


Price  per  pound. 

Immedi- 

ately 

Jan.  i, 

Jan.  i, 

Jan.  i, 

before 

1915- 

1916. 

1917- 

war. 

S.     D. 

S.      D. 

S.    D.    S.     D. 

S.    D. 

Acetanilide 

o  10 

2       O 

6     9-7     o 

2    IO 

Acetylsalicylic  acid  . 

2       O 

6     6 

48     0-50  o 

18     6 

Phenacetin     . 

2     9 

6     6 

60     o 

92     6 

Phenazone 

6     6 

9     6 

75     o 

33     o 

Salol      . 

I    10 

4     9 

47     o 

10     6 

Salicylic  acid 

I       O 

5     o 

20     o 

4     9 

Sodium  salicylate    . 

i     3 

*}     o 

22       0 

5     9 

112      THE  TREASURES  OF  COAL  TAR 

The  situation,  however,  serious  as  it  was,  was 
prevented  from  becoming  disastrous  through  the 
efforts  of  the  academic  chemists  of  the  country. 
The  chemical  laboratories  of  our  Universities  and 
Technical  Colleges  were  converted  into  miniature 
factories,  and  a  supply  of  the  most  necessary  drugs 
was  ensured.  In  time  the  work  of  production 
could  be  taken  up  by  the  regular  manufacturers, 
and  supplies  also  began  to  be  obtained  from  neutral 
countries,  more  especially  Switzerland  and  the 
United  States.  That  a  vast  improvement  in  the 
situation  has  now  taken  place  is  amply  shown  by 
the  prices  quoted  in  the  last  column  of  the  above 
table. 

Not  only  has  the  chemist  garnered  from  the 
boundless  treasures  of  coal  tar  colouring  matters 
which  rival  the  manifold  tints  of  flowers,  but  he 
has  also  evolved  from  that  same  uninviting  source 
substances  which  surpass  in  sweetness  the  sweetest 
of  Nature's  products.  During  the  course  of  a  purely 
scientific  investigation  carried  out  in  the  laboratory 
of  Professor  Ira  Remsen  in  the  Johns  Hopkins 
University  in  America,  Dr  C.  Fahlberg,  in  1879, 
accidentally  discovered  that  one  of  the  compounds 
which  he  had  prepared  possessed  a  remarkably 
sweet  taste  ;  and  he  afterwards  (in  1887)  manu- 
factured the  compound  and  placed  it  on  the  market 
under  the  name  of  saccharine.  The  substance  is 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  113 

derived  from  toluene,  which,  by  successive  treat- 
ment with  concentrated  sulphuric  acid,  chlorine, 
ammonia,  and  oxidising  agents  such  as  perman- 
ganate of  potash,  is  transformed  and  built  up  into 
the  final  product,  benzoic  sulphimide  or  saccharine, 


NH.     It  is  a  white  crystalline  powder 


and  has  a  sweetness  five  hundred  times  greater  than 
that  of  cane  sugar.  How  great  was  the  disaster 
which  threatened  to  overtake  the  cane  and  beet- 
root sugar  industry  as  a  result  of  this  discovery  can 
readily  be  understood.  It  was  as  if  the  story  of 
the  madder  plantations  (p.  81)  was  going  to  be 
retold  for  the  sugar  plantations  of  the  West  Indies 
and  other  parts  of  the  world.  The  whole  machinery 
of  Government  intervention  and  supervision  was 
therefore  set  working,  and  the  general  use  of  sac- 
charine as  a  sweetening  agent  in  articles  of  human 
consumption  was  prohibited,  the  manufacture  of 
the  compound  being  put  under  licence  and  its  sale 
placed  in  the  hands  of  the  druggist.  This  step, 
however,  was  taken  not  merely,  perhaps  not  even 
mainly,  for  the  sake  of  upholding  a  threatened 
industry,  but  from  a  recognition  of  the  fact  that 
saccharine,  unlike  sugar,  has  no  nutritive  value  at 
all,  and  that,  although  it  is  of  importance  as  an 


H4      THE  TREASURES  OF  COAL  TAR 

edulcorant  for  use,  more  especially,  by  those  to 
whom  sugar  is  forbidden  (e.g.  those  suffering  from 
diabetes),  its  uncontrolled  and  unlimited  consump- 
tion is  harmful  and  even  poisonous  to  the  human 
organism.  Saccharine  is  a  medicament,  and  should 
be  treated  as  such. 

Saccharine  is  a  substance  which  dissolves  in 
water  only,  with  difficulty.  By  treating  it  with 
carbonate  of  soda,  however,  it  is  converted  into  a 
sodium  salt  of  saccharine  which,  although  some- 
what less  sweet  than  saccharine  itself,  readily  dis- 
solves in  water.  Similarly,  one  can  obtain  the  readily 
soluble  ammonium  salt,  which  has  the  remarkable 
property  that  it  is  even  sweeter  than  saccharine 
itself,  its  sweetness  being  six  hundred  times  greater 
than  that  of  cane  sugar. 

Formerly  manufactured  almost  exclusively  in 
Germany  and  Switzerland,  preparations  of  sac- 
charine are  now  made  in  England  and  sold  in 
tabloid  form  under  the  name  of  saxin  (Burroughs 
Wellcome  &  Co.). 

While  we  may  regard  the  synthetic  production 
of  colouring  matters  and  drugs  as  being  one  of  the 
greatest  achievements  of  organic  chemistry,  and 
one  which  must  take  an  important  place  in  the 
history  of  human  endeavour  and  of  human  civilisa- 
tion, notable  success  has  also  been  obtained  in  the 
artificial  production  of  those  sweet-smelling  essences 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS]  115 

and  spices  which  in  all  ages  and  by  all  peoples  have 
been  held  in  high  esteem.  In  some  cases  the  chemist 
has  succeeded  in  preparing  substances  which  are 
identical  with  those  to  which  the  odours  of  the 
flowers  are  due ;  in  other  cases  the  synthetic  pro- 
ducts merely  imitate  the  naturally  occurring  per- 
fumes and  spices.  In  some  cases  the  sweet-smelling 
substance  is  built  up,  step  by  step,  from  the  simple 
compounds,  benzene,  toluene,  etc.,  occurring  in 
coal  tar  ;  in  other  cases  these  perfumes  are  obtained 
by  the  transformation  of  naturally  occurring,  com- 
plex compounds,  as  in  the  transformation  of  the 
compound  eugenol  (occurring  in  oil  of  cloves)  into 
vanillin,  the  active  principle  occurring  in  the 
vanilla  bean,  or  of  the  compound  citral  (a  constitu- 
ent of  oil  of  lemon-grass)  into  ionone  or  imitation 
violet.  Not  even  in  the  case  of  the  purely  synthetic 
perfumes,  however,  are  all  the  compounds  derived 
from  coal  tar. 

The  first  of  the  naturally  occurring  perfumes  to  be 
prepared  by  the  chemist — and  first  of  all  by  W.  H. 
Perkin  in  1868 — from  the  products  of  coal  tar 
was  coumarin,  the  fragrant  principle  of  the  Tonka 
bean,  of  the  sweet  woodruff  (Asperula  odorata), 
and  of  certain  clovers,  and  used  in  the  preparation 
of  the  perfumes  known  as  Jockey  Club  and  New- 
Mown  Hay. 

This  compound  can  be  prepared  from  phenol 
or  carbolic  acid.  Phenol  is  first  converted  into 


n6      THE  TREASURES  OF  COAL  TAR 

>H 

salicylic  aldehyde,  |  ,  a  substance  which  is 

'COH 

obviously  closely  related  to  salicylic  acid  (p.  104), 
and  the  salicylic  aldehyde  can  then,  as  W.  H. 
Perkin  showed,  readily  be  transformed,  through 
the  agency  of  acetate  of  soda,  into  coumarin, 

_o  _  co 

Vanillin,    also,    although   generally 


H:CH 


obtained  from  oil  of  cloves,  can  also  be  prepared 
from  toluene. 

To  these  earliest  synthetic  sweet-smelling  sub- 
stances numerous  others  have  since  been  added, 
so  that  from  the  constituents  of  coal  tar  the  main 
odoriferous  principles  of  a  considerable  number 
of  naturally  occurring  essential  oils  and  perfumes 
have  now  been  prepared.  Among  these  one  may 
mention  oil  of  winter-green  (methyl  salicylate,  from 
wood  spirit  and  salicylic  acid),  oil  of  bitter  almonds 
(benzaldehyde,  from  toluene),  hawthorn  blossom 
(anisic  aldehyde,  from  phenol),  oil  of  cinnamon 
(cinnamic  aldehyde,  from  benzaldehyde  or  from 
toluene),  Spircea  ulmaria  or  meadowsweet  (salicylic 
aldehyde,  from  phenol).  Imitation  musk  perfumes 
can  be  prepared  from  toluene ;  and  nitrobenzene, 
as  we  have  already  seen  (p.  53),  was  prepared  at 
an  early  date  and  used,  under  the  name  of  "  essence 
of  mirbane,"  as  a  substitute  for  oil  of  bitter  almonds. 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  117 

Owing  to  the  synthetic  production  of  these  and 
many  other  odoriferous  compounds  at  a  cost  very 
much  less  than  that  of  the  natural  perfumes,  a 
very  great  extension  of  the  use  of  such  substances 
for  the  scenting  of  soaps,  creams,  and  other  toilet 
preparations,  has  taken  place. 

But  if  the  chemist  by  his  transformation  of  the 
constituents  of  coal  tar  has  revolutionised  the  art 
of  dyeing  and  the  science  of  therapeutics,  and  has 
produced  compounds  which  rival  the  perfumes 
of  the  violet  and  the  rose,  he  has  exercised  also 
an  important  influence  on  that  most  practised  of 
all  the  arts,  photography.  The  photographic  dry 
plate  or  film  is  coated  with  a  layer  of  gelatin  con- 
taining a  fine  emulsion  of  the  light-sensitive  salt, 
silver  bromide.  This  salt  is,  however,  not  equally 
sensitive  to  all  the  rays  of  light,  but  is  mainly 
affected  by  blue  and  violet  rays,  while  red  and 
yellow  light  has  practically  no  action.  A  photo- 
graph taken  with  such  a  plate  will,  therefore,  not 
reproduce  a  multi-coloured  object  with  the  proper 
colour-values — the  yellows,  for  example,  will  appear 
darker  than  the  blues.  By  dyeing  the  film  with 
different  coal-tar  dyes,  however,  the  plate  can  be 
made  sensitive  to  light  of  different  colours,  and,  in 
this  way,  "  orthochromatic  "  and  "  panchromatic  " 
plates  have  been  prepared,  the  former  specially 
sensitised  for  green  and  yellow,  the  latter  sensitised 
for  light  of  all  colours. 


n8       THE  TREASURES  OF  COAL  TAR 

But  coal  tar  provides  for  the  needs  of  the  photog- 
rapher not  only  by  furnishing  him  with  colour- 
sensitive  plates,  but  also  by  placing  at  his  service 
a  considerable  number  of  different  "  developers," 
or  substances  by  which  the  latent  photographic 
image  can  be  made  to  appear.  Since  the  character 
of  the  image  depends  to  some  extent  on  the  developer 
employed,  the  intelligent  worker  is  enabled  readily 
to  obtain  the  special  effect  desired. 

Substances  suitable  for  use  as  developers  belong 
to  the  class  known  as  "  reducing  "  substances,  and 
must  contain  two  or  more  hydroxyl  (OH)  groups, 
or  at  least  one  hydroxyl  group  and  one  amino  (NH2) 
group.  Of  such  substances  quite  a  number  have 
been  prepared  from  the  constituents  of  coal  tar, 
and  find  a  more  or  less  extensive  use.  One  of  the 
most  familiar  and  most  widely  used  of  these  is 
"  pyro,"  or  pyrogallic  acid,  or  pyrogallol,  as  it  is 
known  in  chemistry.  This  is  a  derivative  of  benzene 

OH 

containing  three  hydroxyl  groups,  thus  : 


but  although  it  can  be  prepared  synthetically  from 
phenol  (and  therefore  also  from  benzene,  p.  20),  it 
is  usually  prepared  from  gallic  acid,  and  is  there- 
fore not  strictly  to  be  included  among  the  coal-tar 
products.  The  first  true  coal-tar  product  to  be 
used  as  a  photographic  developer  was  hydroquinone, 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  119 

a  substance  which  has  found  much  favour  with 
amateurs  (especially  when  combined  with  other 
developers),  because  of  the  fact  that  it  does  not, 
as  pyro  does,  stain  the  fingers.  Although  this 
compound  had  long  been  known  it  was  not  till 
1880  that  its  use  as  a  photographic  developer  was 
suggested  by  Sir  William  Abney.  First  obtained 
from  quinic  acid,  which  is  found  in  the  medicinal 
extract  of  Peruvian  bark,  it  was  later  discovered 
that  it  could  very  readily  be  prepared  from  aniline, 
and  the  production  of  the  compound  was  thus 
established  on  a  commercially  successful  basis. 
By  treating  aniline  with  a  cold  solution  of  sulphuric 
acid  and  bichromate  of  soda,  it  is  converted  into  a 

X 

compound,  I      jl,  known  as  quinone  (C6H4O2),  and 


this  substance  can  then  be  readily  converted  into 

OH 

n 

hydroquinone,          ,  by  treatment  with  sulphurous 

OH 
acid  or  a  solution  of  sulphur  dioxide  in  water. 

Since  hydroquinone  yields  strong  and  sometimes 
rather  harsh  negatives  it  is  very  frequently  com- 
bined, for  general  use,  with  some  other  developer, 
which  gives  softer  effects.  One  of  the  commonest 


120       THE  TREASURES  OF  COAL  TAR 

of  these  is  "  metol."    When  phenol  is  treated  with 
a  mixture  of  nitric  and  sulphuric  acids,  it  yields 
OH 

para-nitrophenol,         ,  and  when  this  is  "  reduced  " 

N02 

with  tin  and  hydrochloric  acid,  it  is  converted  into 
OH 

para-aminophenol,  ,    just   as   nitrobenzene   is 

NH2 

converted  by  similar  treatment  into  aniline.  The 
salt  of  this  para-aminophenol  with  hydrochloric 
acid  is  the  effective  constituent  of  the  developer 
rodinal.  If  one  replaces  one  of  the  hydrogen  atoms 
of  the  amino-group  by  the  methyl-group  (CH3), 

OH 

one  obtains  the  compound,  ,  which  is  used 

NH-CH3 

as  a  developer  under  the  name  of  scalol.     The  salt 
of  scalol  with  sulphuric  acid  is  the  effective  con- 
stituent of  the  developer  metol.     The  compound 
OH 

1NH'CH3 

,  which  is  isomeric  with  "  scalol,"  forms 


the  basis  of  the  developer  ortol. 


DRUGS  &  PHOTOGRAPHIC  DEVELOPERS  121 

Amidol  is  another  developer  also  derived  from 
phenol,   but    containing  two    amino-groups,   thus: 
OH 

)NH2 

;   and  glycin,  a  somewhat  more  complex 

NH2 

compound,  having  the  formula,  |  ,  is 

\f 

NH'CH2-COOH 

obtained  by  heating  para-aminophenol  (see  above) 
with  monochloracetic  acid,  C1-CH2-COOH. 

The  above  compounds  are  all  derived  from  the 
coal-tar  hydrocarbon  benzene,  but  similar  de- 
velopers have  also  been  derived  from  naphthalene. 
Of  these  the  best  known  is  eikonogen,  a  com- 
pound discovered  by  the  late  Professor  Meldola. 
Its  relation  to  naphthalene  (p.  43)  is  clearly  seen 
from  the  formula, 

NH2 


, 

SOoONal 


In  view  of  the  enormous  development  of  the 
practice  of  photography,  it  will  readily  be  realised 
how  great  is  the  wealth  derived,  in  this  particular 
direction  alone,  from  the  invaluable  coal  tar. 


CHAPTER  X 

EXPLOSIVES 

THE  history  of  civilisation  is,  in  large  measure, 
the  history  of  man's  ability  to  utilise,  control  and 
direct  energy,  and  in  this  respect  the  civilisation  of 
the  nineteenth  and  twentieth  centuries  shows  an 
enormous  advance  on  that  of  all  previous  times. 
And  it  excels  not  only  by  the  amount  of  energy 
which  it  turns  to  useful  account,  but  also  by  the 
degree  to  which  it  can  concentrate  energy ;  for 
material  progress  may  depend  just  as  much,  and 
even  more,  on  the  concentration  of  energy  as  on 
the  actual  amount  of  energy  expended.  Herein 
lies  the  value  of  explosives,  which  represent  highly 
concentrated  forms  of  potential  energy,  capable 
of  being  set  in  motion  at  will,  and  of  producing 
stupendous  results.  In  the  peaceful  progress  of 
civilisation,  no  less  than  in  the  devastation  and  ruin 
of  war,  explosives  have  played  an  all-important 
part,  and  have  made  possible  the  great  engineering 
works  of  the  world,  like  the  Suez  and  Panama 
Canals,  or  the  removal,  in  1885,  of  the  reefs,  known 
as  Hell  Gate,  in  the  channel  of  the  East  River  at 
New  York.  On  this  occasion  over  one  hundred 
122 


EXPLOSIVES  123 

tons  of  explosives,  rackarock  and  dynamite,  were 
employed,  and  millions  of  tons  of  rock  were  dis- 
lodged. But  this  "  blast "  was  small  compared 
with  the  earth-shattering  explosion  of  four  hundred 
and  fifty  tons  of  high  explosive  which  preceded 
the  capture  of  the  Messines  Ridge  by  the  British 
Army  on  the  morning  of  June  7th,  1917. 

An  explosive  may  be  denned  as  a  substance  or 
mixture,  solid  or  liquid,  capable  of  undergoing 
extremely  rapid  combustion  or  decomposition,  with 
production  of  gaseous  substances  which  occupy 
a  volume  it  may  be  ten  or  twelve  thousand  times 
as  great  as  that  of  the  explosive  itself.  In  the  case 
of  gunpowder,  cordite,  and  other  propellants  (low 
explosives),  there  is  a  rapid  combustion  of  the 
explosive,  but  in  the  case  of  high  explosives — to 
which  class  all  the  coal-tar  explosives  belong — the 
molecules  of  the  compound  are  in  a  somewhat  un- 
stable condition,  and,  when  subjected  to  a  suit- 
able shock,  undergo  decomposition  into  more  stable 
substances.  This  decomposition  is  generally  initi- 
ated by  means  of  a  "  detonator,"  or  substance 
which  is,  comparatively,  very  sensitive  to  shock, 
and  the  "  explosive  wave  "  which  is  set  up  is  trans- 
mitted with  a  very  great  velocity — amounting  in 
some  cases  to  more  than  four  miles  per  second — 
and  so  causes  an  almost  instantaneous  decom- 
position of  the  explosive. 

Although,  the   first   real   explosive,    black   gun- 


124      THE  TREASURES  OF  COAL  TAR 

powder,  was  discovered  (by  Roger  Bacon)  in  the 
thirteenth  century,  no  further  advance  in  the 
chemistry  of  explosives  was  made  until  the  nine- 
teenth century.  In  that  century  a  number  of  new 
and  very  powerful  explosives  were  introduced,  and 
although  two  of  the  most  important  of  these — gun- 
cotton  and  nitroglycerin  (dynamite) — are  not  de- 
rived from  coal  tar,  derivatives  of  this  have, 
in  recent  times,  begun  to  play  a  most  important 
part,  especially  in  connection  with  naval  and 
military  operations. 

The  first  coal-tar  explosive  to  be  obtained  was 
picric  acid.  Although  discovered  in  1771,  it  was  not 
till  1843  that  it  was  prepared  from  phenol,  by  the 
action  of  a  mixture  of  sulphuric  and  nitric  acids. 
The  substance  is  now  also  prepared  from  benzene, 
from  which  compound,  also,  phenol  itself  is  now 
largely  produced  owing  to  the  greatly  increased 
demand  for  this  substance. 

Picric  acid,  or  tri-nitro-phenol  (to  give  it  its 
chemical  name),  is  derived  from  phenol  by  the 
replacement  of  three  hydrogen  atoms  by  three 
nitro-groups  (NO2),  as  is  represented  by  the  formula 
C6H2(N02)3-OH,  or 


EXPLOSIVES  125 

It  is  a  lemon-yellow  coloured  crystalline  substance 
which  found  its  first  use,  and  still  finds  use  to  a 
slight  extent,  as  a  dye  for  silk  and  wool ;  and  it 
readily  stains  the  skin  also  of  a  yellow  colour.  The 
explosive  decomposition  of  picric  acid  can  be 
effected  by  means  of  a  suitable  powerful  detonator, 
but,  under  ordinary  conditions,  it  is  a  quite  stable 
substance  which  melts  at  a  temperature  of  122°  C. 
(252-6°  F.),  and,  when  strongly  heated,  burns  with 
production  of  a  large  amount  of  black  smoke.  This 
stability  and  insensitiveness  to  ordinary  shocks 
and  blows  are  clearly  a  great  advantage  from  the 
point  of  view  of  safety  in  handling ;  and  the  sub- 
stance picric  acid  was  adopted  in  1885  by  the 
French  Government  as  a  high  explosive  for  filling 
shells,  under  the  name  of  melinite  (from  the  honey- 
like  appearance  of  the  molten  compound),  and  some 
years  later  by  the  British  Government,  under  the 
name  of  lyddite  (from  Lydd,  in  Kent,  where  its 
explosive  properties  were  tested).  Other  countries, 
also,  have  adopted  picric  acid  as  a  high  explosive  for 
military  purposes,  and  it  forms  the  sole  or  main  con- 
stituent of  the  explosives  pertite  (Italy),  shimosite 
(Japan),  and  Dunnite  (United  States).  Some  idea 
of  the  power  of  this  explosive  will  be  gained  from 
the  statement  that  when  one  pound  of  picric  acid 
is  exploded  it  liberates  an  amount  of  energy  equal 
to  that  required  to  raise  a  weight  of  over  a  ton  to 
a  height  of  more  than  a  hundred  yards. 


126      THE  TREASURES  OF  COAL  TAR 

From  meta-cresol  (p.  44),  tri-nitro-cresol  (known 
in  France  as  cresylite),  similar  to  tri-nitro-phenol 
(picric  acid),  has  also  been  prepared.  It  is  less 
powerful  than  picric  acid,  but  has  sometimes  been 
used  for  mixing  with  the  latter  in  order  to  lower 
its  melting-point,  and  so  render  it  less  inconvenient 
to  manipulate.  Its  ammonium  salt  was  formerly 
used  as  a  high  explosive  by  Austria  under  the  name 
of  ecrasite. 

Although  picric  acid  itself  is  comparatively  in- 
sensitive to  shock,  it  has  the  disadvantage  that  it 
forms  compounds  (picrates)  with  metals,  such  as 
lead,  copper,  iron,  etc.,  which  are  much  more 
sensitive  to  shock  and  which  may  cause  premature 
explosion  of  the  shell.  Hence  the  necessity  for 
coating  the  interior  of  the  shell  with  a  varnish. 
The  priming  composition  known  as  Brugere  powder 
is  a  mixture  of  ammonium  picrate  and  saltpetre 
(potassium  nitrate). 

From  phenol  and  methyl  chloride  there  is  pre- 
pared the  compound  anisole,  C6H5-OCH3 ;  and  by 
nitrating  this  one  obtains  tri-nitro-anisole,  an 
explosive  which  has  recently  been  used  by  the 
Germans  for  filling  bombs. 

From  the  hydrocarbons  of  coal-tar,  also,  powerful 
explosives  can  be  prepared.  Of  these  the  most  im- 
portant is  undoubtedly  tri-nitro-toluene,  obtained 
by  nitrating  toluene  with  a  mixture  of  concentrated 


EXPLOSIVES  127 

sulphuric  and  nitric  acids.  It  forms  a  white  crystal- 
line substance  which  melts  at  a  much  lower  tem- 
perature (81°  C.  or  177-8°  F.)  than  picric  acid,  is 
even  less  sensitive  to  mechanical  shock  and  rough 
usage  than  this  explosive,  and  does  not  form 
dangerously  explosive  salts  with  metals.  Although 
it  acts  as  a  powerful  explosive  when  exploded  by 
means  of  a  suitable  detonator,  it  is  a  comparatively 
stable  substance,  so  stable,  indeed,  and  safe  to 
handle,  that  it  does  not  come  under  the  provisions 
of  the  Explosives  Act  with  respect  to  its  manu- 
facture, transport,  and  storage.  The  advantages 
which  tri-nitro-toluene  thus  possesses  over  picric 
acid  led  to  its  adoption  by  Germany  in  1902,  and 
by  other  Governments  at  a  later  date,  as  a  high 
explosive  for  filling  shells ;  and  for  this  purpose 
it  has,  although  a  less  powerful  explosive  than 
picric  acid,  largely  displaced  that  compound.  It 
has  also  to  a  large  extent  taken  the  place  of  gun- 
cotton  as  the  explosive  filling  for  torpedoes  and 
submarine  mines.  In  the  British  Services  it  is 
known  as  trotyl,  or  as  T.N.T.  When  ignited, 
trotyl,  like  picric  acid,  burns  without  explosion  as 
a  rule,  but  disastrous  explosions  have  also  occurred 
through  the  combustion  of  large  quantities  of  the 
compound,  especially  in  presence  of  ammonium 
nitrate. 

The  combustion  of  T.N.T. ,  as  well  as  its  decom- 
position by  detonation,  are   accompanied   by  the 


128      THE  TREASURES  OF  COAL  TAR 

production  of  dense  black  clouds  of  carbonaceous 
or  sooty  matter,  owing  to  there  being  insufficient 
oxygen  in  the  compound  to  combine  with  all  the 
carbon  present ;  and  this  has  led  to  the  nicknames 
of  "  Coal  boxes "  and  "  Jack  Johnsons  "  being 
applied  to  the  shells  filled  with  this  explosive.  In 
order  to  secure  more  perfect  combustion  and,  at 
the  same  time,  to  reduce  the  amount  of  trotyl 
required,  ammonium  nitrate  (NH4NO3),  a  substance 
containing  an  excess  of  oxygen,  is  frequently  added. 
In  this  way  the  British  service  high  explosive 
amatol,  a  mixture  of  trotyl  and  ammonium  nitrate, 
is  obtained. 

Not  only  is  trotyl  used  as  an  explosive  by  itself, 
but  it  also  forms  a  constituent  of  a  number  of  com- 
posite explosives.  Thus,  the  Austrian  explosive 
ammonal  is  a  mixture  of  trotyl  (30  per  cent.), 
ammonium  nitrate  (47  per  cent.),  aluminium  powder 
(22  per  cent.),  and  charcoal  (i  per  cent.).  By  the 
combustion  of  the  aluminium  powder  the  tempera- 
ture of  the  explosion  is  considerably  raised  and 
the  explosive  force  consequently  increased.  Trotyl 
also  forms  a  constituent  of  the  Belgian  high  ex- 
plosive macarite  (trotyl  and  lead  nitrate),  and 
of  the  blasting  explosives  rexite  and  Withnell 
powder. 

Di-nitro-toluene,  in  which  only  two  NO2-groups 
are  present,  is  also  used  to  some  extent  in  the  pre- 
paration of  composite  explosives  for  blasting  pur- 


EXPLOSIVES  129 

poses.  Of  these  the  most  important  are  the  various 
cheddites  (so  called  from  Chedde,  in  France,  where 
they  are  manufactured),  consisting,  for  example,  of 
ammonium  perchlorate  and  di-nitro-toluene,  mixed 
with  a  small  amount  of  castor  oil,  to  diminish 
the  sensitiveness  of  the  mixture  to  friction.  Other 
similar  mixtures  are  also  prepared  under  the  name 
cheddite. 

Although  of  less  importance  than  tri-nitro-toluene, 
the  nitro-derivatives  of  benzene  are  also  used  to 
a  considerable  extent,  more  especially  in  the  pro- 
duction of  composite  blasting  explosives.  Even 
nitrobenzene  (C6H5-N02),  itself,  although  not  an 
explosive,  is  used  as  a  combustible  material  in  such 
explosives  as  rackarock  (potassium  chlorate  and 
nitrobenzene)  and  petrofracteur  (potassium  chlor- 
ate, nitrobenzene,  potassium  nitrate,  and  antimony 
sulphide).  These  explosives  belong  to  the  class 
known  as  Sprengel  explosives,  in  which  the  oxygen 
producer  (potassium  chlorate,  etc.)  and  the  com- 
bustible substance  (nitrobenzene,  etc.)  are  kept 
separate  and  mixed  just  when  and  where  the  ex- 
plosive is  to  be  used.  The  employment  of  such 
explosives  is  prohibited  in  Great  Britain. 

Di-nitro-benzene,  also,  although  it  can  be  de- 
tonated only  with  difficulty  and  is  not  used  as  an 
explosive  by  itself,  forms  a  constituent  of  certain 
composite  explosives,  such  as  securite  (di-nitro- 
benzene  and  ammonium  nitrate) ;  and  chlor- 
i 


130      THE  TREASURES  OF  COAL  TAR 

di-nitro-benzene  (or  di-nitro-benzene  in  which  a 
hydrogen  atom  has  been  replaced  by  chlorine), 
when  mixed  with  ammonium  nitrate,  yields  the 
powerful  blasting  explosive  roburite.  Tri-nitro- 
benzene,  on  the  other  hand,  is  an  explosive  which 
is  more  powerful  than  either  picric  acid  or  tri-nitro- 
toluene,  but  owing  to  the  greater  difficulty  and 
expense  of  its  manufacture  has  not  so  far  come 
into  general  use.1 

Other  nitro-derivatives  of  coal-tar  hydrocarbons, 
although  of  less  importance  than  those  already 
mentioned,  have  also  been  proposed  for  use  as 
explosives,  and  have  even  been  adopted  to  some 
extent.  Of  these  one  may  mention  di-nitro-naph- 

1  The  nitro-derivatives  of  benzene,  and  to  a  somewhat  less 
extent  tri-nitro-toluene,  exercise  a  very  marked  toxic  action, 
to  which  some  individuals  are  more  susceptible  than  others. 
Absorption  of  these  compounds  into  the  system,  which  takes 
place  more  especially  through  the  skin,  may  give  rise  to  derma- 
titis, toxic  gastritis,  toxic  jaundice,  and  finally  death.  It  is, 
therefore,  of  the  highest  importance  not  only  that  all  factories 
in  which  T.N.T.  (the  most  important  of  the  nitro-compounds 
used  at  the  present  time)  is  made  shall  be  efficiently  ventilated, 
but  the  greatest  cleanliness  also  must  be  observed  on  the  part 
of  the  workers  so  as,  more  especially,  to  prevent  continued 
contact  of  T.N.T.  with  the  skin.  The  handling  with  the  un- 
covered hands  of  T.N.T.  or  of  articles  which  have  been  in  con- 
tact with  T.N.T.  should  be  as  far  as  possible  avoided.  As  a 
measure  of  precaution  it  has  been  laid  down  as  a  rule  by  the 
Minister  of  Munitions,  in  respect  of  workers  in  T.N.T.  factories, 
that  "  no  person  shall  be  employed  for  more  than  a  fortnight 
without  an  equal  period  of  work  at  a  process  not  involving 
contact  with  T.N.T.,  or  an  equal  period  of  absence  from  work 
unless  such  employment  has  been  approved  by  the  Medical 
Officer." 


EXPLOSIVES  131 

thalene,  which  is  employed  as  a  constituent  of  the 
blasting  explosive  known  as  favierite  (ammonium 
nitrate  and  di-nitro-naphthalene)  and  of  schneiderite 
(ammonium  nitrate,  88  parts  ;  di-nitro-naphthalene, 
ii  parts ;  resin,  i  part),  used  by  the  French  for 
filling  high-explosive  shells. 

It  may  be  mentioned  that  these  ammonium 
nitrate  explosives,  e.g.  securite,  roburite,  favierite, 
are  of  importance  on  account  of  the  fact  that  they 
are  "  safety  explosives  "  ;  that  is  to  say,  they  do 
not,  on  explosion,  ignite  mixtures  of  fire-damp 
and  air,  and  can  therefore  be  used  for  blasting 
purposes  in  coal-mines. 

From  aniline  and  other  amino-compounds  valu- 
able explosives  have  also  been  prepared.  Thus,  in 
recent  years,  there  has  been  obtained,  by  the  nitra- 
tion of  aniline,  the  compound  which  goes  by  the 
common  name  of  tetranyl  (tetra-nitro-aniline), 
which  promises  to  find  more  extensive  application, 
especially  as  a  primer ;  and  by  the  nitration  of 

diphenylamine,   <^      y— NH— <^        \,  there  has 

been  obtained  the  compound  hexa-nitro-diphenyl- 
amine — a  substance  first  introduced  as  the  dye 
aurantia — which  has  recently  been  used  to  some 
extent  by  Germany  for  the  filling  of  bombs.  Di- 
phenylamine is  itself  used  as  a  stabiliser  in 
military  smokeless  powders. 
The  discharge  of  the  various  coal-tar  explosives, 


132       THE  TREASURES  OF  COAL  TAR 

now  known  in  considerable  numbers,  is  brought 
about,  as  has  already  been  mentioned,  not  by 
ignition  (as  in  the  case  of  gunpowder  and  cordite), 
but  by  the  detonation  of  a  more  sensitive  explosive. 
Until  recently,  fulminate  of  mercury  (from  mer- 
cury, nitric  acid,  and  alcohol),  alone  or  mixed  with 
potassium  chlorate,  was  practically  the  only  de- 
tonator employed ;  but  this  detonator  is  both 
dangerous  to  handle  and  expensive  to  manufacture. 
The  discovery,  therefore,  that  the  amount  of  ful- 
minate required  could  be  very  greatly  diminished 
if  mixed  with  tri-nitro-toluene,  or  with  picric  acid, 
was  one  which  has  had  results  of  great  importance. 
Still  better  are  the  results  obtained  by  means  of 
a  mixture  of  "  tetryl "  (tri-nitro-phenyl-methyl- 

CH3  N02 

N 

nitramine,  NO2/ NNO2),  fulminate  of  mercury,  and 

N02 

potassium  chlorate. 

Such  are  the  main  achievements  of  the  chemist 
in  producing  those  powerful  engines  of  civilisation, 
explosives,  from  the  constituents  of  coal  tar.  But 
year  by  year  new  compounds  are  being  added  to 
the  armouries  of  the  nations  and  the  magazine  of 
the  engineer ;  and  the  successes  of  the  past  are  but 
an  earnest  of  still  greater  triumphs  in  the  future. 


INDEX 


Abney,  Sir  William,  119 
Acetanilide,  103 
Acetyl-salicylic  acid,  105 
Acid  yellow,  71 
Acridine  dyes,  84 
Acri-flavine,  84,  no 
Adrenaline,  107 
Alcohol,  ethyl,  37 

„        methyl,  37 
Alcohols,  37 
Algol  red,  86 

,,      yellow,  86 
Aliphatic  compounds,  38 
Alizarin,  81 

,,       annual  production  of, 

83 

,,        bordeaux,  84 
,,        brown,  84 
,,        orange,  84 
,,        preparation  of,  82 
„        red  S,  84 
Alizarine  delphinol,  86 
Alypine,  107 
Amatol,  128 
Amidol,  121 
Ammonal,  128 
Ammonia,  sulphate  of,  5 
Anaesthesine,  106 
Aniline,  53,  54 
„        black,  79 
blue,  58 
purple,  53 
„        red,  55 
,,        yellow,  70,  71 
Anisic  aldehyde,  116 
Anthracene,  21,  43 

blue,  84 
,,  dyes,  80 

oils,  17 


Anthraquinone,  80 
Antife  brine,  102 
Antipyretics,  101 
Antipyrine,  101 
Aromatic  compounds,  40 
Arsamin,  109 
Asperula  odorata,  115 
Aspirin,  105 
Atom,  32 
Atoxyl,  109 
Auxochromes,  65 
Azo-dyes,  70 

B 

Bacon,  Roger,  124 

von  Baeyer,  Adolf,  93 

Bechamp,  53 

Becher,  J.  J.,  2 

Beehive  ovens,  5 

Benzaldehyde,  67,  116 

Benzene,  18,  20 

„        constitution  of,  41 
,,         poisonous    action    of 
nitro-derivatives  of, 
130 

Benzidine,  75 

Benzoic  acid,  68 

Benzol,  18,  22 

„       commercial,  19 
,,       from  coal  gas,  18 
,,       rectification  of,  18 

Berzelius,  32 

Beta-eucaine,  107 

Biebrich  scarlet,  74 

Bismarck  brown,  72 

Bitter  almond  oil,  67 

Bohn,  R.,  85 

Bottinger,  75 

Brilliant  green,  67 

Browning,  Dr,  no 

133 


134      THE  TREASURES  OF  COAL  TAR 


Brugere  powder,  126 
By-product-recovery  ovens,  7,  8 


Carbolic  acid,  19,  20,  21,  24,  42 

„      oils,  17 
Cheddites,  129 
Chemical  structure,  36 
Chloranthrene  blue,  86 
Chlor-dinitro-benzene,  129 
Chlorine,  production  of,  98 
Chloroform,  106 
Chromogen,  65 
Chromophors,  65 
Chrysoidine,  72 
Ciba  dyes,  99 
Cinnamic  aldehyde,  116 
Clark,  Sir  James,  52 
"  Coal  boxes,"  128 
Coal,     destructive    distillation 

of,  3 

„        products  of,  3,  ii 
Coal  tar,  amounts  of  constitu- 
ents, 20 
,,      ,,   annual    consumption 

of,  2 
„       „   applications  of ,  in  raw 

state,  22 

„  „  distillation  of,  13,  15 
,,  ,,  nature  of,  10,  13,  15 
„  „  production  of,  i,  3,  5, 

12 

,,       ,,   solvents,  use  of,  24 
Coke,   production  of,   in   Bee- 
hive ovens,  5,  7 
„       production    of,    in  By- 
product-recovery 
ovens,  7,  8 
Colin,  8 1 

Colour  and  constitution,  64 
Congo  red,  76 
Constant  proportions,  law  of, 

38 

Constitution  of  molecules,  39 
Contact  process,  97 
CoppSe  oven,  8 
Cordite,  123 

Cotton  dyes,  direct,  75,  76 
Coumarin,  115 


Creosote,  26 

„         annual  production  of, 

28 
,,         antiseptic    properties 

of,  29 
„          applications  of,  26/29 

oils,  17,  20 
Cresolin,  26 

Cresols,  20,  25,  26,  44,  106 
Cresylic  acid,  25 
Cr6sylite,  126 

Cross  Dye  Black  F.N.G.,  88 
Crystal  violet,  64 

D 

Detonators,  132 
Developers,  photographic,  118 
Diamine  green,  76 
Diesel  engines,  29 
Di-nitro-benzene,  129 
Di-nitro-naphthalene,  130 
Di-nitro-toluene,  128 
Drugs,  synthetic,  100 

,,  „          price  of,  ill 

Dunnite,  125 
Dye  drugs,  108 

Dyes,  coal  tar,  annual    produc- 
tion of,  51 

,,       „       „'  industrial     pro- 
duction of,  49 

„       „       „    value  of,  51 

„     from  coal  tar,  48 

„     imports    of,    into    Great 
Britain,  52 

„     vat,  80,  88 


Ecrasite,  126 

Ehrlich,  Paul,  108 

Eikonogen,  121 

Empirin,  105 

Essence  of  Mir  bane,  116 

Ethane,  35 

Ethylene,  35 

„        series,  35 

Explosives,  122 

high,  123 
low,  123 


INDEX 


135 


Fadeless  fabrics,  85 
Fahlberg,  C.,  112 
Faraday,  Michael,  52 
Fatty  compounds,  38 
Favierite,  131 
Fischer,  Emil,  60 
Otto,  60 

Flavanthrene,  86 
Formulae,  33 

„         diagrammatic 
(graphic),  34 

Frankland,  Sir  Edward,  34 
Friedlander,  99 
Fuchsine,  55 


Gas,  coal,  production  of,  3,  4 

,,     illuminating,  3 
Gasoline,  35 
Girard,  58 
Glycin,  121 
Graebe,  81 
Green,  A.  G.,  78,  79 
Griess,  Peter,  70 
Gunpowder,  123 

H 

Hawthorn  blossom,  116. 
Heavy  oils,  20 
Helindon  yellow,  86 
Heumann,  93,  96 
Hexa-nitro-diphenylamine,  131 
Hofmann,  A.W.,  14,  52,  56 

„       violets,  58 
Hydrocarbon,  saturated,  34 

„  unsaturated,  35 

Hydroquinone,  118 


Ice  colours,  79 

Indanthrene  blue,  85 
„  dyes,  84 
,,  green,  86 

„  red,  86 

,,          yellow,  86 


Indican,  91 

Indigo,  90 

„      dye  industry,  91 

,,       synthetic,  British,  96 

Indigotin  (indigo  blue),  91 
„          synthesis  of,  94,  96 

Ingrain  dyes,  78 

Intermediates,  49 

Iodides,  37 

lonone,  115 

Isatis  tinctoria,  go 

Isomerism,  38 


"  Jack  Johnsons,"  128 

Jeyes'  Fluid,  26 

Jockey  Club  Perfume,  115 

K 

Kairine,  101 

von  Kekule,  August,  36,  41 
Khaki  Brown  C.,  88 
„      yellow  C.,  88 
Khar  si  van,  no 
Knorr,  Ludwig,  101 


de  Laire,  58 

Lakes,  83 

Law  of  Constant  Proportions, 

38 

Leuco-compounds,  88,  89 
Liebermann,  81 
von   Liebig,    Justus,    52,    101, 

106 

Light  oil,  17 
Lucigen  lamp,  29 
Lyddite,  125 
Lyons  blue,  58 
Lysol,  26 


^acarite,  128 
Mackintosh,  14 
ladder,  81     ^ 
Magenta,  55 
"Malachite  green,[,67 
Mai  de  caderas,  109 


136      THE  TREASURES  OF  COAL  TAR 

Mansfield,   Charles    Blachford 


Mauve,  53 

Meadowsweet  perfume,  116 

Medlock,  56 

Meldola,  R.,  121 

Melinite,  125 

Methane,  34 

,,         series,  35 
Methyl  green,  64 

„       salicylate,  116 

„       violet,  64 
Metol,  1 20 
Middle  oils,  19 
Mir  bane,  essence  of,  53 
Mitscherlich,  52 
Molecular  architecture,  31 
Molecule,  33 

Molecules,  constitution  of  39 
Murdoch,  William,  3 
Murex  brandaris,  99 
,,      trunculus,  99 
Musk  perfume,  imitation,  116 


N 


Naphtha,  18,  22 
Naphthalene,  19,  21,  26,  43 
Naphthol,  73 
Naphthylamine,  73 
New  Fuchsine  process,  62 
New  Mown  Hay  perfume,  115 
Nicholson  E.  C.,  45,  56,  58    ' 
Nicholson  s  blue,  58 
Nitrobenzene,  53,  54 
Novocaine,  107 


Oil  of  bitter  almonds,  116 
„   „  cinnamon,  116 
,,   „  wintergreen,  116 

Oleum,  97 

Oriol  yellow,  78 

Orthochromatic  plates,  117 

Ortho-nitrotoluene,  59 

Ortho-toluidine,  60 

Ortol,  120 


Panchromatic  plates,  117 
Paracelsus,  100 
Paraffin  wax,  35 
Para-nitraniline  red,  78 
Para-nitrotoluene  59 
Para-red,  78 
Para-rosaniline,  61 
Para-toluidine,  60 
Pentabrom-phenol,  106 
Perfumes,  synthetic,  114 
Perkin    Sir  W.  H.,  45,  53,  71, 

79,82,115 
Pertite,  125 
Petrofracteur.  129 
Petrol,  35 
Phenacetine,  104 
Phenazone,  101 
Phenol,  20,  21,  42,  105 
Phenyl-hydrazine,  101 
Photographic  chemicals,  117 
Picric  acid,  124 
Piroplasmosis.  109 
Pitch,  29 
Ponceaux,  74 
Primuline,  78 

„          brown,  78 

„          orange,  78 

„          red,  78 
Prince  Consort,  52 
Propane,  35 
Propylene,  35 
Purpurin,  84 
Pyramidone,  102 
Pyrogallic  acid,  118 
Pyrogallol,  118 


Q 


Quinoline,  101 


Rackarock,  129 
Refined  tars,  29 
Regepyrin,  105 
Remsen,  Ira,  112 
Rexite,  128 
Rhodamines,  87 


INDEX 


137 


Robiquet,  81 

Roburite,  130 

Rodinal,  120 

Roofing  felt,  30 

Royal  College  of  Chemistry,  52 

Rubia  tinctoria,  81 


Saccharine,  112 
Safety  explosives,  131 
Saffranines,  87 
Salicylic  acid,  104 

„         aldehyde,  116 
Salol,  105 

Salvarsan  ("  606  "),  no 
Saxin,  114 
Scalol,  120 
Schneiderite,  131 
Securite,  129 
Serle,  Henry,  2 
Shimosite,  125 
Simpson,  Maule,  and  Nicholson, 

.56 

Simpson,  Sir  James,  106 

Sleeping  sickness,  109 

Soamin,  109 

Spivosa  ulmaria,  116 

Spirits  of  wine,  37 

Spirochcete  palli da,  no 

Sprengel  explosives,  129 

Stovaine,  107 

Sulphide  dyes,  87 

Sulphur  dyes,  87 

Sulphuric  acid,  manufacture  of, 

97 

Sundour  fabrics,  85 
Suprarenine,  108 
Symbols,  32 
Syphilis,  no 


Takamine,  107 
Tar,  refined,  29 
Tetrabrom-ortho-cresol,  106 


Tetranyl,  131 
Tetryl,  132 

Timber,  creosoting  of,  14,  27 
„       preservation     ("  pick- 


,  ,       poisonous  action  of  ,  1  30 
Tolidine,  75 
Toluene,  18,  20,  21,  42 
Tri-nitro-anisole,  126 
Tri-nitro-benzene,  130 
Tri-nitro-cresol,  126 
Tri-nitro-phenol,  124 
Tri-nitro-toluene,  126 
Triphenyl-methane,  60 
Tropaeoline  O.,  73 
Tropaeolines,  72 
Trotyl,  127 
Trypa-flavine,  84 
Trypan  blue,  109 

„       red,  109 

Trypanosoma  gambiense,  log 
Tyrian  purple,  53,  99 


Valency,  doctrine  of,  34 
Vanillin,  115 
Vaseline,  35 
Vat  dyes,  80,  88 
Verguin,  55 
Victoria  green,  67 
Violet,  imitation,  115 

W 

Water  blue,  58 
Withnell  powder,  128 
Woad,  90 
Wohler,  44 
Wood  spirit,  37 


Xylene,  18,  19 


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